but what is much more difficult to determine is the reason for their decoration,

B over A for subject(the reason) verb agreement(is)

gmatmachoman wrote: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

Why B is better over A?

OG 10

The three points in the end of the sentence is considered a LIST

what is much more difficult to determine is.....parallelism has to be maintained.

FightWithGMAT wrote:
The three points in the end of the sentence is considered a LIST

what is much more difficult to determine is.....parallelism has to be maintained.

FYI, you are misunderstanding the concept of parallelism.

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gmatmachoman wrote:
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

Why B is better over A?

OG 10
The three points in the end of the sentence is considered a LIST
--------------------------

B is better bcoz points in the list are all singular. If they were plural, we would use plural - eg:
what is much more difficult to determine are the reasons and effects for their decoration on sale

Does it help.

I think you guys are confusing subject/verb agreement with parallelism. I believe you can make the list parallel even with varying counts as long as the list parallels in verb/adjective/noun and several other criteria.