B. I had the flu last week I stayed in bed for four days.
Explanation:
<u>A fused sentence occurs when two independent clauses are incorrectly joined without any single punctuation mark</u>. 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: <em>I had the flu last week</em><em>, so</em><em> I stayed in bed for four days. </em>