c
Hexadeuterobenzene is an isotope of benzene where all hydrogen atoms are replaced by deuterium.

In the nitration of benzene mechanism nitronium ion formed by nitric acid.

Removing of hydrogen don't take participate in rate determination step so kinetic isotope effect is absent.

The rate of nitration of Deuterated benzene is same that of the benzene when same sets of nitrating agent is used in both cases.