Which sentence is a run-on sentence

A opossums move very slowly and are caught easily.

B coyotes are nomads-they usually travel alone

C ostrich eggs are huge and weigh about three pounds each

D pigs are intelligent they can be taught tricks

The answer is D, this is a run-on sentence because it is formed by two complete sentences ("pigs are intelligent" and "they can be taught tricks") but the expression lacks the proper punctuation signs to make it easy to read.

In other words, a run-on sentence is missing a proper link between two main ideas, or independent sentences that become "fused" to each other.

D pigs are intelligent they can be taught tricks