The correct answer is C) slow the swing using moderate rudder. The helm command "Check her" is used to slow down the swing or turning motion of the vessel. This is typically done by applying moderate rudder, rather than hard over rudder which would stop the swing abruptly. Applying moderate rudder allows the vessel to gradually reduce its rate of turn and come back to a steady course. The other options are incorrect because: A) stopping the swing using hard over rudder would be too abrupt, B) testing the steering control is a different command, and D) reading the compass heading is not the purpose of the "Check her" command.