Which of the following sentences is grammatically correct?

The chosen sentence is grammatically correct because it uses the appropriate form of the pronoun "whom" in the context of the question being asked. "Whom" is used as the object of a verb or preposition, and in this case, it follows the preposition "to," making it the correct choice.

The sentence structure reflects proper grammatical rules: "To whom do I owe the pleasure..." clearly indicates that the speaker is inquiring about the recipient of the gratitude. Additionally, the second part of the sentence ("I find your singing delightful") maintains grammatical integrity by correctly describing the subject's appreciation of the singer's performance.

In contrast, the other options either incorrectly use "who" as the object of the sentence or misplace the preposition. For example, using "who" would be suitable if it were the subject of the sentence, but since the construction involves a preposition, "whom" is necessary for correctness. The choice of pronouns must adhere to standard rules of English grammar regarding subject and object pronouns, making this selection the most accurate under those guidelines.