Okay, when something is understated, there's actually more of it than what is recorded. To check this, you need to work from the source docs toward the F/S, or trace (for completeness), because there will be more source info than what is actually recorded. Overstatement works the other direction, by vouching from the F/S to the source docs you can trace (for existence) of the supporting source docs (for which some will be missing in an overstatement).
Using this logic, I would answer D to the above question because we're looking for an understatement of sales. Since sales actually occur the moment goods leave the entity's custody, shipping docs are a great way to see what was actually sold, and since we're looking for an understatement of sales (some sales haven't been recorded) we should test if there are shipments for which there are no sales invoices, and therefore didn't end up in the sales journal.