Which signal shall a power-driven vessel sound when making way in fog?

A power-driven vessel that is underway in reduced visibility, such as fog, must follow the sound signal requirements established by the International Regulations for Preventing Collisions at Sea (COLREGs). Specifically, when making way through fog, the correct sound signal is to sound one prolonged blast every two minutes. This signal is used to communicate the presence and position of the vessel to others in the vicinity, thereby helping to prevent collisions in low-visibility conditions.

The rationale behind using a prolonged blast is that it is clear and distinguishable, even in challenging auditory environments like fog, where other sounds may be muffled. This type of signal is a standardized way for power-driven vessels to indicate their status and alert nearby vessels of their presence while they are in motion.

Other signals provided in the options do not align with the regulations for a power-driven vessel making way in fog. Therefore, the choice of one prolonged blast every two minutes is essential for ensuring safety and maintaining adherence to maritime communication standards during periods of reduced visibility.