Answer:
B
Explanation:
A comma splice is most commonly two simple sentences separated by a comma. Usually a period is better.
A: works with a comma. While exercising can be fun is a subordinate clause. The comma is correct. Answer not A.
C: Same as A. The clause before the comma is a subordinate clause. That's fine. The answer is not C.
D: but usually does require a comma before it. A comma works here. Answer not D.
B is a comma splice. Two simple sentences separated by a comma.