The correct answer is A. Donna likes biology, it is her favorite subject.
Explanation:
According to grammar rules, it is correct to use comma to separate different items in a list or in the case of compound sentences that link two ideas that have the same level of importance through conjunction such as and, or, but, so, because in this case, the comma is placed before the conjunction.
However, it is not correct to use a comma to link two complete sentences as the correct form is to use a semicolon (;). This type of mistake occurs in the case of "Donna likes biology, it is her favorite subject", because in this there are two independent clauses or complete sentence "Donna likes biology" and "it is her favorite subject", which is a grammatical mistake known as comma split. Therefore, the sentence that has an error in comma usage is "Donna likes biology, it is her favorite subject".
