Which sound signal alerts nearby vessels that assistance is needed?

The choice indicating repeated signals of distress is correct because it specifically refers to the established maritime communication protocol that signals to other vessels that immediate assistance is required. Under the COLREGs and international maritime conventions, signaling distress is a critical way to convey that a vessel is in need of help.

Repeated signals of distress can include various forms of communication, such as repeated action of sirens, fog horns, or other sound-making devices. This form of signaling ensures that nearby vessels are aware of a dire situation and can respond appropriately to assist. It effectively captures attention over distance and through environmental noise, prompting a response from other mariners who might be positioned to provide aid.

The other options, while they are important sound signals, do not specifically denote a need for assistance. A series of long blasts is typically used to signify a maneuvering action or a vessel's intention rather than a distress call. A continuous ringing bell is more often associated with signaling in harbor environments or indicating a navigational warning. A single blast for attention serves as a general alert but does not specifically imply that assistance is urgently needed. Thus, repeated signals of distress is the most accurate representation of a call for help on the water.