For this case we have that by definition, the article is the word that goes before the noun. A definite article or also called a determinant, is one that refers to a thing that is known and that can be identified.
In the plural we have:
Feminine: Las
Male: Los
In singular we have:
Female: La
Male: El
In this case we have the word "clase":
It is a singular word in feminine, so the definite article to use is "La":
