Fill in the blank with the correct option listed below:
I am not sure _______ this book belongs to.
a) who
b) whom
c) whose
d) which

Hint: A pronoun is a function word that is used in place of a noun or noun phrase. For example- he, she, it, they, who.

Complete answer:
In the given question, we have to find which of the pronouns given to us in the options correctly fit in the sentence. A relative pronoun shall be put in the blank.
Let us thus analyze the options given to us in this question-
Option (a.), 'who', means ‘(relative pronoun) the person or people that.’
Therefore, option (a.) is incorrect as who is used when the subject of the sentence can be replaced by a subject pronoun.
Option (b.), ‘whom', means ‘Him; her; them (used as a relative pronoun to refer to a previously mentioned person or people.)’
Therefore, option (b.) is correct as it is used when the object of the sentence can be replaced by an object pronoun, like in this sentence, where the object is the book.
Option (c.), ‘whose', refers to ‘(formerly proscribed) Of which, belonging to which; used as a relative pronoun.’
Therefore, option (c.) is incorrect as this is the possessive form of who.
Option (d.), ‘which', refers to a word that is used for non-living things. Therefore, option (d.) is incorrect.

Note:
 In this question, it will be helpful for the student to recall the concept of relative pronouns. A relative pronoun is a pronoun (as `that' or `which' or `who') that introduces a relative clause referring to some antecedent.