Take a look at the sentence again:
Laura and Diane bang on Boen's door when they need heavy furniture moved,
after they have clogged the garbage disposal, and __________.
The blank requires a subordinate clause. You chose having discovered a spider in the bathtub, which is a gerund phrase and thus causes a parallelism error. Before you continue, review the rules for parallel structure.
Go back to the sentence to try again.