One and two short blast signals must be sounded on inland waters when __________.

The correct answer is that one and two short blast signals must be sounded on inland waters when two power-driven vessels are in sight of one another and are meeting at a distance of one-half mile. This specific signaling is part of the sound signaling rules outlined in the Navigation Rules, which ensure that vessels can communicate their intentions clearly to avoid collisions.

In this scenario, one short blast indicates "I am altering my course to starboard," while two short blasts indicate "I am altering my course to port." It is essential for vessels meeting head-on or nearly so to make these signals to maintain safe navigation and avoid misunderstandings about each vessel's intended maneuvers.

The other options do not align with this specific requirement. For example, overtaking scenarios, entering a harbor, or approaching a bridge have distinct signaling protocols that do not involve the one and two short blast combinations described in the question. By knowing and applying the correct signaling for meeting situations, mariners can ensure safer interactions on the water.