The run on sentence is D. Snails move very slowly, a banana slug can move 6 inches in two hours.
A run-on sentence is a sentence that has at least two independent clauses which are not properly punctuated.
When two independent clauses are connected only by a comma, it is a run-on sentence that is called a comma-splice.
Run-on sentences can be corrected with the use of conjunctions (for, and, nor, but, or, yet, so)
Snails move very slowly, and a banana slug can move 6 inches in two hours. (corrected run-on sentence)