Concept:The possessive pronoun "whose" is needed before a noun to show ownership.
Explanation:The blank is followed by the noun "voice".
Therefore, we need a possessive word meaning "belonging to whom".
"Who" is a subject pronoun and "whom" is an object pronoun.
Neither "who" nor "whom" can describe a noun like "voice".
"Who's" is a contraction of "who is" or "who has".
For example, "Who's coming?" means "Who is coming?"
Only "whose" acts as the possessive form of "who".
It is used to ask or state that something belongs to someone.
So the sentence becomes: "It is not easy to identify whose voice it was."
Answer:D. whose