Right answer:
A. el armario
El armario translates into the closet So armario is a masculine and singular noun. On the other hand, el is the definite article that matches the masculine and singular form. Recall that the definite article is used when thinking that the reader or listener knows which particular thing or person we are talking about. Therefore:
IN SPANISH:
Voy a poner mis libros en el armario
IN ENGLISH:
I'm going to put my books in the closet