Both answers are correct.
A is correct because you used the verb ‘are’ this is the verb ‘be’ in the plural form in the present simple and ‘classes’ is the plural noun of ‘class’.
B is also correct because you used the verb ‘is’ which is the singular form of the verb ‘be’ in the present simple and ‘class’ is a singular noun.
If you had said; the classes is dirty that would be incorrect because ‘classes’ is in the plural and ‘is’ is singular.
The class are dirty would be incorrect also because ‘class‘ is singular and ‘are’ is plural.
