The correct option is C if
The rules for changing an interrogative sentence in direct speech to reported speech say that we should use "if" or "whether" if the sentence begins with a helping or auxiliary verb.
As the given sentence begins with the auxiliary verb "is", "if" in option D is the correct answer.
"What" in option A, "which" in option B and "where" in option C are all incorrect as an interrogative word (such as "what, when, which, where etc.") is used if the interrogative sentence in direct speech does not begin with an auxiliary verb.
Thus, option D is the correct answer.