If the trailer begins to sway noticeably at highway speed with no clear cause, what is a reasonable first response?

Correct answer to question 138 (Combination Vehicles): Ease off the accelerator and prepare to pull over to inspect the trailer, tires, and load once safe to do so

Why

Unexplained sway can indicate a problem such as a shifting load, tire issue, or suspension fault, so easing off the accelerator and stopping to investigate is safer than continuing or trying to force a fix while moving.