Where is . . . . . . . . box I lent you last month? A. a B. an C. the D. no article

The correct answer is: no article.

The article “a” or “an” is used before a singular noun that refers to a general or indefinite thing. In this case, the box is a specific box that the speaker has lent to the listener, so it is not appropriate to use an article.

The article “the” is used before a singular noun that refers to a specific or definite thing. In this case, the box is a specific box that the speaker has lent to the listener, so it is not appropriate to use “the” either.

Therefore, the correct answer is “no article”.