Which distress signal is recognized at sea, consisting of a series of repeated signals?

The signal recognized at sea that consists of a series of repeated signals is the SOS signal. This distress signal is characterized by the Morse code representation of "SOS" ("...---..."), which is transmitted as three short dots, three long dashes, and three short dots. Its universal recognition and simplicity make it an effective means of alerting others to a vessel's distress, particularly in situations where verbal communication may not be feasible.

In maritime practice, the SOS signal is intended to convey urgency and requires immediate attention. Many vessels and crew members are trained to recognize this signal as a call for help, leading to prompt responses from nearby vessels or rescue services.

The other choices reflect various terms associated with distress signals but do not specifically denote the series of repeated signals recognized globally as SOS. For example, "mayday" is also a recognized distress signal but is communicated verbally rather than through a series of repeated signals. Therefore, "SOS" is the correct answer that captures the essence of the question regarding repeated signals at sea.