Topic: Pronoun (সর্বনাম),

Best Answer Explained in Below-:

In the sentence "I have read the book - you lent me," the use of "that" as a relative pronoun introduces a defining relative clause, which is crucial for clarifying which book is being referenced. The phrase can be rewritten as "I have read the book that you lent me." Here, "that" specifies the particular book among potentially many others, making it essential for understanding the meaning of the sentence.

Let's briefly consider the other options. "Whom" is used to refer to people, making it inappropriate since "book" is not a person. "Whose" indicates possession and is used for ownership, which does not apply here because the sentence does not need to convey ownership of the book, only its identity. Finally, "what" is generally used in questions or when referring to a thing or idea in a general sense, which does not fit the context of specifying a particular book.

Thus, "that" is the most suitable choice to create a clear, grammatically correct sentence that effectively communicates which book the speaker is discussing.