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.

C is not a sentence at all.

That leaves A and B.

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.

The run-on sentence is option B) "After the store don't run around."

The group of words that is a run-on sentence is option B) "After the store don't run around."

A run-on sentence is a sentence that consists of two or more independent clauses (complete thoughts) that are joined together without proper punctuation or conjunctions. In option B, there are two independent clauses, "After the store" and "don't run around," that are not properly connected. To fix the run-on sentence, you can either separate the two independent clauses into separate sentences or use appropriate punctuation or conjunctions to join them correctly.