When is a flip-flop said to be transparent?

Answer - B. when the Q output follows the input

Flip-flop is said to be transparent when the Q output follows the input.

Solution

A flip flop is transparent when the Q output is the same as the input. Transparent means you can see from both sides whatever is on another side. 

Therefore, the answer to the question  When is a flip-flop said to be transparent will be when the Q output follows the input. Hence, Option (B) is the correct answer.