The relative pronoun 'whom' is used for humans. To refer to the noun 'book', we require the relative pronoun 'that'.
The correct sentence is: This is the book that I wanted to buy.
Hence, option (c) is the answer.
Note: Relative pronouns: A relative pronoun is used to connect a clause or phrase to a noun or pronoun. The clause modifies, or describes, the noun. The most common relative pronouns are who, whom, whose, which, and that. For example: She is the girl who received the award for bravery from our Home Minister.
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