Answer:
B. I had the flu last week I stayed in bed for four days.
Step-by-step explanation:
A fused sentence occurs when two independent clauses are incorrectly joined without any single punctuation mark. As a result, the clauses should be separated by either a semicolon (as in option D) or a coma plus a coordinating conjunction, such as: I had the flu last week, so I stayed in bed for four days.