The experiment failed ______ the scientists had overlooked a crucial variable.
Correct: A
The original sentence ("The experiment failed, the scientists had overlooked a crucial variable.") is a comma splice because two independent clauses are joined only by a comma. A semicolon (;) is one correct way to join two closely related independent clauses. Other valid corrections would be to use a period to create two separate sentences, or a comma followed by a coordinating conjunction (e.g., "and," "for," "so").