¿hora es la clase? qué cuál cuáles

The right option that could fill the blank is:

- ¿A qué hora es la clase?

Explanation:

The words "qué" and "cuál" have two very marked uses that can be taken into account when selecting them within a sentence, "cuál" or its plural "cuáles" is usually used when selecting an option from a homogeneous group, for example, the seller of an ice cream shop wants to know the flavor of ice cream that the buyer wants, therefore, in Spanish he would ask the question with "cuál" and it is called the homogeneous group since all are flavors, in the case of the hour is not so, since although all are hours, the option is undetermined, that is, a heterogeneous group, therefore, the word that is usually used to ask for the time is "qué".