Let's look at the difference btwn the words "with" and "of"
WITH - is a preposition word used to describe associative relationships.
OF - is a preposition word used to describe a spatial or logical relationship.
In this case, since both are prepositions, it is imperative to define the subject and what modifying phrases are related to the subject.
Let's break this sentence down to its components
The Anasazi settlements....(STOP) - "settlements" is your subject
...at Chaco Canyon were built...(STOP) - this piece tells you a location and also provides the verb phrase "were built"
...on a spectacular scale...(STOP) - what is your subject? is it scale? no, it is settlements...
Now which option should you choose? It is either option A or B. Why? What sounds better? Settlements were built..."of" or "with"? In this case, you are trying to describe an associative relationship - so using the preposition "with" is the correct choice.
...with more than 75 carefully engineered structures...(STOP) - based on the above and that there is no difference btwn A or B on this part, sounds good so far
...of up to 600 rooms each...(STOP) - this part of the sentence is a preposition relating the "structures" having a spatial relationship to the "rooms"
..., (STOP) - notice the comma. when do you use commas in sentences? It is usually used to list things, introduce a phrase, or take a pause. In this sentence, you are using it to introduce a modifying phrase
...were connected by a complex regional system of roads. (STOP) - This is the deciding factor. Do you need the verb "were"? What is the verb referring to? This part of the sentence is referring to the rooms being connected by a complex regional system of roads. If you are wondering, well what about the initial subject "settlement". If you said "the settlements were built...were connected" - you would need the conjuctive word "and". However, since you used a comma, you are introducing a phrase to modify the subject "rooms". In this case, it is not appropriate to use the verb bc you are introducing the modifying phrase "connected by a complex regional system of roads" to describe/modify the subject each room. So you don't need the verb "were". Which leaves option B as the better choice.
