C is not a sentence at all.
That leaves A and B.
Pick Which group of words is a run-on sentence:
A) Don't run around.
B) After the store don't run around. C)After the store.
2 answers
Actually, none of these groups of words is a run-on sentence. A is a complete sentence. B is, too, although it's really incomplete. A mother might say to her child, "After you have been to the store, don't run around." That is not a run-on sentence.