In ancient times, Nubia was the principal corridor where there were cu

A) where there were cultural influences transmitted
'where there were' is Ackward.
It also does not clearly indicate that the region was a conduit for
the transmission

(B) through which cultural influences were transmitted
Correct. It implies correctly that Nubia was the conduit enabling the transmission

(C) where there was a transmission of cultural influences
'a transmission of cultural influences' implies that it was transmitted by a single transmission. This changes the meaning.

(D) for the transmitting of cultural influences
'for the transmitting of' is not the correct Idiom - 'for the transmission of' would have been better

(E) which was transmitting cultural influences
This implies corridor is transmitting cultural influences, whereas corridor is just a conduit.