Explanation:

The correct answer is option C, "which."

The word or phrase that best completes the sentence is "which."

"A letterbox is a narrow opening in a door through which mail is delivered."

Explanation:

In this sentence, the word "which" is used to introduce a relative clause that provides additional information about the noun "opening." The relative clause "mail is delivered" describes the purpose or function of the opening.

In English grammar, relative clauses are introduced by relative pronouns or relative adverbs. In this case, "which" is a relative pronoun that refers to the noun "opening" and connects the relative clause to the main clause of the sentence.

The word "which" is used to refer to things or objects. In this sentence, it refers to the "opening" in the door. The relative clause "mail is delivered" provides more information about the opening and explains its purpose.

Other options:

- Option A, "it," is not the best choice because it does not properly connect the relative clause to the main clause of the sentence. The sentence would be grammatically incorrect if "it" were used in this context.
- Option B, "that," could also be a possible choice, but "which" is more commonly used in this type of sentence structure.
- Option D, "where," does not properly connect the relative clause to the main clause. "Where" is typically used to refer to a place, not an opening in a door.

Overall, option C, "which," is the best choice because it properly introduces the relative clause and connects it to the main clause, providing additional information about the opening in the door.