Final answer:
Waiting for the mailman is the most likely experience when the mail delivery schedule is unpredictable. With no set time, mail arrival is uncertain, leading to potential waits at the mailbox.
Step-by-step explanation:
When the mailman's schedule is unpredictable, what you might experience when checking the mailbox for mail is D) Waiting for the mailman to show up. Unpredictable schedules can lead to not knowing exactly when the mail will be delivered, causing individuals to potentially wait for the delivery person to arrive. This contrast with predictable schedules where mail delivery times are consistent and known in advance.