Correct Answer: The librarian asked me which book I was taking.

Solution : The correct choice is the fourth option.

Explanation:
The indirect form of a sentence involves reporting or paraphrasing what someone else has said. In the indirect form, the reporting verb, tense, and pronouns may change. The structure typically involves transforming the direct speech into a more indirect or reported style.
Narration conversion:

• The reporting verb "said" changes to "asked" to reflect the question.
• The second person pronoun "you" changes to the first person pronoun "I".
• The present continuous tense "are taking" changes to the past continuous tense "was taking" in the indirect form.

Therefore, the indirect form would be: The librarian asked me which book I was taking.