What should you do when the roads are wet, icy, or snow-covered?

Answer: C — Turn the retarder off

When driving in wet, icy, or snow-covered conditions, it's essential to prioritize safety and control of the vehicle. Turning off the retarder is correct because a retarder is designed to help slow down the vehicle by using engine compression or other means, and it can potentially lead to loss of traction on slippery surfaces. Engaging the retarder in these conditions may cause the wheels to lock up or skid, increasing the danger while driving. In adverse weather, it’s vital to maintain optimal control over the vehicle by using gentle braking and steering. Being cautious in these situations helps prevent accidents and ensures safer travel. The other options are not advisable in such conditions as they could impair vehicle control or safety.