The period when the great painted caves at Lascaux and Altamira were

The subject is "what is much more difficult to determine", and therefore singular verb "is" is correct.

The period when the great painted caves at Lascaux and Altamira were occupied by Upper Paleolithic people has been established by carbon-14 dating，but what is much more difficult to determine are the reason for their decoration，the use to which primitive people put the caves，and the meaning of the magnificently depicted animals．


first sentence the subject is - period- verb has is correct


second clause starting with but- subject is - what is much more difficult to determine- the entire phrase so verb are is wrong as phrase is always singular

(A) has been established by carbon-14 dating，but what is much more difficult to determine are
(B) has been established by carbon-14 dating，but what is much more difficult to determine is
(C) have been established by carbon-14 dating，but what is much more difficult to determine is
(D) have been established by carbon-14 dating，but what is much more difficult to determine are
(E) are established by carbon-14 dating，but that which is much more difficult to determine is

A. has been established by carbon dating, but what is much more difficult to determine are
(1) SV agreement: “….what is much more difficult to determine IS the use…” relative clause is the subject

B. have been established by carbon dating, but what is much more difficult to determine are
(1) SV agreement: “The period….have been….” – the when-clause is a super long modifier of “period”

C. have been established by carbon dating, but that which is much more difficult to determine is
(1) SV agreement: “The period….have been….” – the when-clause is a super long modifier of “period”

D. has been established by carbon dating, but what is much more difficult to determine is
correct

E. are established by carbon dating, but that which is much more difficult to determine is
(1) SV agreement: “The period….are been….” – the when-clause is a super long modifier of “period”

The period when the great painted caves at Lascaux and Altamira were occupied by Upper Paleolithic people has been established by carbon-14 dating, but what is much more difficult to determine are the reason for their decoration, the use to which primitive people put the caves, and the meaning of the magnificently depicted animals.

Let's reread it without modifiers.
The period when the great painted caves at Lascaux and Altamira were occupied by Upper Paleolithic people has been established by carbon-14 dating, but what is much more difficult to determine are the reason for their decoration, the use to which primitive people put the caves, and the meaning of the magnificently depicted animals.

(A) has been established by carbon-14 dating, but what is much more difficult to determine are
(B) has been established by carbon-14 dating, but what is much more difficult to determine is
(C) have been established by carbon-14 dating, but what is much more difficult to determine is
(D) have been established by carbon-14 dating, but what is much more difficult to determine are
(E) are established by carbon-14 dating, but that which is much more difficult to determine is

The subject is singular.
Hence C, D, and E are out.
for A, ...what is... determine ARE...
Inconsistency in helping verb.

B is correct.

The period when the great painted caves at Lascaux and Altamira were occupied by Upper Paleolithic people has been established by carbon-14 dating, but what is much more difficult to determine are the reason for their decoration, the use to which primitive people put the caves, and the meaning of the magnificently depicted animals.

(A) has been established by carbon-14 dating, but what is much more difficult to determine are -> "are" is incorrect, we need singular verb.
(B) has been established by carbon-14 dating, but what is much more difficult to determine is -> It is better. Let's keep it.
(C) have been established by carbon-14 dating, but what is much more difficult to determine is -> "the period" subject is singular, so "have" is incorrect to use.
(D) have been established by carbon-14 dating, but what is much more difficult to determine are -> "the period" subject is singular, so "have" is incorrect to use.
(E) are established by carbon-14 dating, but that which is much more difficult to determine is -> -> "the period" subject is singular, so "are" is incorrect to use.

So, I think B.

Well, this is a fun and tricky one.

This is a good example of why it's important to "see the big picture" in SC. Although the verb "are" is technically correct because of the plural subject that follows the verb (decoration + use + meaning = plural), it is not possible to use "are" in this case, because we have boxed ourselves in by using the phrase "what is _____ " in all of the answer choices, which cannot be followed by "are" due to a lack of parallelism.

What is ____ is _____ = correct
What is ____ are ___ = incorrect

Another arguably correct choice would be "what are ____ are ____", but let's face it: in the real world, no one talks like that. Why? Because unlike most sentences, where the subject precedes the verb, in this particular sentence the subject follows the verb, so it doesn't sound right to use the plural verb "are"—even though it's technically correct.

For example:

The minimum amount of force required to break a concrete block with one's hand is well-known. What is more difficult to determine is the precise angle, technique, and speed required to avoid injury.

sounds better than

The minimum amount of force required to break a concrete block with one's hand is well-known. What are more difficult to determine are the precise angle, technique, and speed required to avoid injury.

...even though the 2nd version is more technically correct.

Hi mcelroytutoring

Thank you for your clear explanation!

Hi experts

Could you please explain the restrictive clause "the use to which primitive people put the caves"
I have learnt "put something to use" in which "use" is a verb

But how come here the use, which is a noun can be used here in a put's verbal phrase??

Hello!
But what would be logical reason of placing "is" in that clause " what is much more difficult.." against "are"?

Hello!
But what would be logical reason of placing "is" in that clause " what is much more difficult.." against "are"?

Let's wait for others to chip in as well....you do realize Ekansh that all the five options use exactly the same structure " what is much more difficult.."..right?

Yes. But some other question can have an option with 'are' in front of what. So what is the reason for placing 'is'?