You are driving a motor vehicle with a manual gearbox. What is the logical thing to do so that the cruise control system can contribute to a fuel-efficient driving style?

You answered correctly. For a car with a manual gearbox, selecting the right gear is essential for fuel-efficient driving, even when using cruise control. Cruise control maintains a set speed, but it cannot change gears for you in a manual car. If you are in the wrong gear—such as driving in a low gear at high speed or a high gear at low speed—you may use more fuel than necessary because the engine runs inefficiently. By choosing the correct gear for your speed and driving conditions, you keep the engine operating at optimal efficiency, allowing cruise control to maintain speed without unnecessary fuel consumption. This is why "selecting the right gear" is the logical action for economy when using cruise control with a manual gearbox.