Is the example a complete sentence, sentence fragment, or run-on sentence?

Outside the city limits, the houses are spaced farther apart.

A.

run-on sentence

B.

complete sentence

C.

sentence fragment

This is a complete sentence. you have a subject (houses) and a verb (are spaced), which means it isn't a fragment that needs more information. it isn't a run on sentence either--there aren't multiple conjunctions or misused commas.