The correct answer is C) timing the movement of the rudder from hard over to hard over. The test of a towing vessel's steering gear control system should include a visual inspection of the steering linkage (A), verification that the rudder angle indicator shows the actual position of the rudder(s) (B), and a test from the alternative power supply, if installed (D). However, timing the movement of the rudder from hard over to hard over is not a required component of this test, as it is not a regulatory requirement for towing vessels. The reason C is the correct answer is that the test of the steering gear control system is focused on ensuring the system is functioning properly and can be controlled from the operating station, rather than specifically measuring the rudder response time.