Which of the following is an example of a grammatically correct sentence?

The choice highlighting "I cannot remember to whom I lent the book" is grammatically correct due to its proper use of the word "whom," which serves as the object of the preposition "to." In English grammar, "who" is used as a subject, while "whom" is used as an object. Therefore, in this context, where the sentence is referencing the recipient of the action (lending the book), "whom" is appropriate.

Additionally, the structure of the sentence is correct as it maintains clarity and follows the conventional syntax expected in formal writing. This contributes to effective communication, ensuring the reader understands that the speaker is referring to the person receiving the book.

The other sentences contain errors in the use of "who" and "whom," or they present awkward or non-standard constructions that deviate from grammatical norms. In particular, using "to who" in option A misapplies the pronoun, and option D incorrectly utilizes "whom" as a subject. Therefore, option C stands out as the only sentence that adheres to the rules of English grammar appropriately.