Topic: Pronoun (সর্বনাম),

Best Answer Explained in Below-:

The phrase "I don't know - book it is." suggests a context where someone is trying to identify or establish possession in relation to a book, thus requiring a word that indicates ownership. In the options provided, the only one that fits this context grammatically and semantically is "c) whose."

To break it down, "whose" is a possessive pronoun used to inquire about the ownership of something, as in "Whose book is this?" It functions to ask to whom a particular object (in this case, a book) belongs.

In contrast:
- A) "who's" is a contraction of "who is" or "who has," which does not pertain to possession.
- B) "who" is a subject pronoun that would not fit in a possessive context.
- D) "whom he's" is incorrect because it mixes a relative pronoun with a pronoun-verb combination awkwardly without serving a meaningful role in the sentence.

Thus, "whose" succinctly fits the need to express ownership of the book, making it the only accurate answer in this context.