What signal is made when approaching a bend or obstruction in a channel which may obscure another vessel?

When approaching a bend or obstruction in a channel that may obscure another vessel, the appropriate signal to use is a prolonged blast of the horn. This is known as a "one prolonged blast," which serves to alert other vessels in the vicinity of your presence and intentions, especially in areas where visibility may be limited. The sound of the blast is intended to communicate to other nearby operators that you are approaching a potentially hazardous situation due to the obstruction and to ensure that they are aware of your approach, enhancing safety on the water.

In general, the lengths and types of sound signals are standardized to create clarity and prevent collisions, and a single prolonged blast is specifically designated for situations where a vessel may be at risk of encroaching upon the path of another vessel due to an obstruction. The other options do not apply in this maritime communication context, as they serve different purposes in signaling maritime maneuvers and intentions.