Answer:
semicolon
Step-by-step explanation:
Compound sentences consist of two independent clauses which can stand on their own and have related ideas. Therefore, both clauses can be joined either by a comma followed by a coordinating conjunction, or by a semicolon.
The rest of the options are incorrect because a period does not join clauses - it separates sentences; a colon can join two independent clauses when the first one introduces a quotation, an example or a list; and a comma alone does not divide independent clauses.