B. It has a comma splice and needs a conjunction.
The error in this sentence is that it has two independent clauses separated by a comma, but no conjunction to connect them. This is called a comma splice. The best way to fix it would be to add a conjunction such as "and" or "but" between the two clauses, to show the relationship between them.
Example: Marin blew bubbles, and Chris blew up balloons.
Alternatively, you can separate the two independent clauses into two distinct sentences by using a period or a semicolon.
Example: Marin blew bubbles. Chris blew up balloons.