Take a look at the sentence again:
Ironing a clean shirt, __________, and buying a pack of gum for fresh breath
were the tasks Aaron completed before the first day of spring semester.
The blank requires a gerund phrase. You chose making sure that he had printed a course schedule, which is a gerund phrase—but with an adjective clause attached. You thus have a parallelism error. Before you continue, review the rules for parallel structure.
Go back to the sentence to try again.