Which of the following choices is a run-on sentence?
a. most beetles can fly some beetles can't.
b. some beetles that can't fly are called ground beetles.
c. the hard forewings are called the elytra.
d. many beetles that can't fly have the two elytra fused together.

A sentence becomes a run-on sentence when it is composed of two or more clauses which are not separated by an appropriate punctuation or conjunction. From the given sentences above, the sentence that is a run-on sentence is option A. Most beetles can fly some beetles can't. This sentence contains two clauses and should be separated by a semi-colon. Most beetles can fly; some beetles can't.