The Anasazi settlements..

Can an expert provide more insight to why "were" is to be removed from the last clause?

Ah-ha, another forum shed some light into breaking down the structure of the sentence. "of up to 600 rooms each" is a non-essential clause, and by removing that clause, it is clear that were should be removed.

Question - doesn't "with more than 75 carefully engineered structures" refers to scale and not to the settlements? Comma + preposition should refer to the noun right before the comma, correct?

A is correct...!

Anyone that wanna help?

Question - doesn't "with more than 75 carefully engineered structures" refers to scale and not to the settlements? Comma + preposition should refer to the noun right before the comma, correct?

The correct answer is B, Find the explanations below:

The Anasazi settlements at Chaco Canyon were built on a spectacular scale with more than 75 carefully engineered structures, of up to 600 rooms each, were connected by a complex regional system of roads.

A. with more than 75 carefully engineered structures, of up to 600 rooms each, were
This choice is a run on, if you remove all modifiers etc. you are left with: the settlements were built with structures were connected.that's bad. Run-0n sentence: A sentence incorrectly formed of tqo main clauses joined without proper punctuation or a proper connecting word, such as a subordinator.

B. with more than 75 carefully engineered structures, of up to 600 rooms each,
This is the right answer it uses a nonessential modifier set off by commas ('of up to 600 rooms each'), which, if eliminated, yields the intact and legitimate sentence...carefully engineered structures, connected by... (with another nonessential modifier).

C. of more than 75 carefully engineered structures of up to 600 rooms, each that had been
- 'scale of' doesn't make sense
- you can't say 'each that had...' (can't follow 'each' with a relative pronoun - if you're going to use a relative pronoun, it has to come directly after the thing it's trying to modify)
- no justification for using the past perfect ('had been') - that verb, if there's a verb there at all, should be in the simple past (the same tense as everything else in the sentence, because everything described in the sentence is contemporaneous)
- it doesn't make sense to use 'each' AFTER the comma, because it's not true that each structure was connected with a road system. instead, the road system connected all of the structures with each other, which is nowhere close to the same thing. (having 'each' BEFORE the comma makes sense, because it's actually true that each of the structures comprised up to 600 rooms.)
analogy:
the USA comprises 50 states, each of which is united by a federal government--> wrong (the implication is that each state has its own federal government)
the USA comprises 50 states, all of which are united by a federal government --> correct
the USA comprises 50 states, (all) united by a federal government --> correct, whether you have 'all' or not

D. of more than 75 carefully engineered structures of up to 600 rooms and with each
- 'scale of' doesn't make sense
- the use of AND sets up ostensible parallelism, but the two structures given aren't parallel (one starts with of and the other with with)

E. of more than 75 carefully engineered structures of up to 600 rooms each had been
choice e is also a run-on sentence (you'll see this if you reduce it to its 'skeleton', a la choice a)
Disclaimer: I have borrowed some of explanation from Ron's Post.

A really tough one..can jeopardize verbal..

Final solution at one place:

Important: The purpose of this post (and all the other posts by me) is to give a complete solution to all GMAT-Prep Verbal questions at one place. Sometimes students have to wade through dozens of posts to get to the final answer. My posts will give one complete and crisp solution required to arrive at the correct answer by eliminating the wrong ones. Some of the content in these posts may have been taken from various other sources (discussion forums).

The Anasazi settlements at Chaco Canyon were built on a spectacular scale with more than 75 carefully engineered structures, of up to 600 rooms each, were connected by a complex regional system of roads.
(A) with more than 75 carefully engineered structures, of up to 600 rooms each, were
(B) with more than 75 carefully engineered structures, of up to 600 rooms each,
(C) of more than 75 carefully engineered structures of up to 600 rooms, each that had been
(D) of more than 75 carefully engineered structures of up to 600 rooms and with each
(E) of more than 75 carefully engineered structures of up to 600 rooms, each had been

In this construction, ‘scale of’ is the wrong idiom. ‘Scale of’ is correctly used in constructions such as: ‘on a scale of 0-10’. This eliminates C, D, and E.

In A, if we remove the nonessential part between the commas, we are left with:

The Anasazi settlements at Chaco Canyon were built on a spectacular scale with more than 75 carefully engineered structures were connected by a complex regional system of roads.

In A, we can’t use ‘were’ … ‘were’ without a connecting word (such as ‘and / or / but / that etc. depending upon the meaning’). In this sentence, the ‘structures’ were ‘connected’, so we surely need some connector (preferably ‘that’) between ‘structures’ and ‘were’. Or else, A becomes a run-on sentence. It tries to have two main verbs without using any sort of subordinating element. If you take out modifiers, adjectives, etc., you're left with the following: the settlements were built with structures were connected. UGH!

Similarly, E is a run-on sentence.

In B, if we remove the nonessential part between the commas, we are left with:

The Anasazi settlements at Chaco Canyon were built on a spectacular scale with more than 75 carefully engineered structures, connected by a complex regional system of roads.

In the sentence above, the last part is a non-essential modifier. Perfect.

In C, D, and E, ‘scale of’ is the wrong idiom.
Also, in C, we can’t use the past perfect ('had been') as the events don’t have a time lag.