What is a run-on sentence?

a. two independent clauses run together without punctuation or a conjunction

b. two independent clauses joined by a comma without a conjunction

c. a broken-off part of a sentence

d. an independent clause containing two dependent clauses?

I know the answer isn't C or D. A or B both seem likely until you see that B involves a 'comma without a conjunction,' either way you should never put a comma before 'and'; I am not sure why but I am pretty sure this only applies in a formal document. So your answer is A.