A is the correct answer choice.

Answer choices B and C are incorrect, because the plural verb "have" (at the beginning of the underlined section) does not agree with the sentence's singular subject "the period". Answer choice E also begins incorrectly with a plural verb ("are").

Answer choices D and E are incorrect because the singular verb "is" (at the end of the underlined section) does not agree with the plural subject of the sentence's second clause. This is a case of what we call "verb precedes subject". Contrary to where we expect to find it, the subject of this clause actually comes after the verb, at the end of the sentence, as a list of three items describing the features of the magnificently depicted animals: "the use to which...,the reason for..., and the meaning of...". This list of three items, separated by commas and ending with "and" before the last item, form a plural subject.

How is it possible to see here that the subject comes after the verb? The most "common sense" understanding of this sentence is that "the use..., the reason, and the meaning of the magnificently depicted animals are much more difficult to determine". Hence - the list of three is the subject.

two things:

1. The period...............has
--> This kills B, C, and E.

2. If a clause is used as the subject of a verb, that verb is always singular. A clause is singular.

So, what is much more important is...

--> This kills A, and B.

Answer is D.

Absolutely right on E - tunnel vision will get you in the end.

Re: 2, I guess we disagree on what is the subject here: I read the sentence as an ass-backwards transformation of: "the use..., the reason..., the meaning ARE what is more difficult to determine", with the "what is" clause acting as the object, not the subject.

OA?

two things to note:
1. The period... has (singular)
2. what is much more to diff to determine... is (noun phrase)

I would opt for D...


whats.. OA.. .

IMO D .

I choose opt A

I go with D...OA please