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.