Answer:
d. semicolon
Step-by-step explanation:
A run-on sentence is a sentence consisting of two or more independent clauses (clauses that express a complete thought) that are not connected by a period, semicolon or a conjunction and that, instead, are connected improperly with a comma or that are not connected with any other punctuation.
When a run-on sentence, then, has two independent clauses that are closely connected in meaning, it can be corrected by joining the two clauses with a semicolon because one of the uses of semicolons is to link two independent clauses whose ideas are closely related and that are not linked with a conjunction like and, but, or, nor, for, yet, or so.