What does the signal of one prolonged blast plus two short blasts indicate?

The signal of one prolonged blast followed by two short blasts is a maritime signal indicating that a power-driven vessel is making its presence known to others, especially in restricted visibility, and is typically used to communicate its intentions. This specific signal is recognized internationally and is employed by vessels to alert others in the vicinity that they are moving, which helps in maintaining safe navigation.

The option suggesting that this signal is used by sailing vessels is incorrect because it is primarily associated with power-driven vessels. The nuances of signaling are crucial for vessel communication, particularly in foggy or low-visibility conditions, to prevent collisions and ensure everyone's safety on the water.