What is the definite article for clase

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":

La clase

La clase