The correct option is A aren't they
Question tags transform a declarative or imperative sentence into
interrogative. They are formed using the verb (auxiliary/helping verb) and pronoun (that indicates
the subject) from the sentence.
The pronoun derived from the given sentence is 'they' ('they' refers to "these exercises") and 'are' is the verb.
Hence the question tag formed will be: aren't they (verb + pronoun).
The question tag is negative (i.e., aren't) when the sentence is positive (are) and vice versa.
Hence option A is correct.
Option B: The verb 'isn't' is singular whereas the verb in the statement 'are' is plural. Hence B is incorrect.
Option C is incorrect because 'don't' is not the verb used in the sentence.
Similarly, 'won't' is not the correct form in the sentence, hence option D is also incorrect.