ans should be D

No need to use past perfect so A and B are out
use of prior to is awkward in C and E

Answer is D.But 'first proof' of what.Shouldn't it be explicitly mentioned as it is in C?

There is no other thing first proof can refer to other than the PT. Thats why IMO its ok even though we don't have the theorem in place.

Hello Everyone,

Any better explanation why D and not C ?
I know prior to was unidiomatic, but isn't choice D ambiguous with no reference to subject of first proof (the theorem).
I think I am missing some basic point here and need clarification.

Regards,

Though the Pythagorean theorem is attributed to Pythagoras, the first proof had not been completed before the year 300 BC, when it appeared in Euclid's Elements.

Read it like this when you wanto to solve it.

A. the first proof had not been completed before the year
B. it had not been proven to be true before the year
C. the first proof of the theorem was not completed prior to
D. the first proof was not completed until
E. it was not proven to be true prior to

Now dont you think there is no need to explain as shown in "C" and that "D" is the better option