Which of the following is a recognized form of distress signal?

Continuous horn blasts are a recognized form of distress signal according to maritime conventions. This signal is typically used to attract attention in emergency situations where someone needs help. The use of continuous sound, such as a horn, indicates the urgency of the situation and communicates to other vessels or nearby individuals that assistance is required. These blasts are distinctive and can carry over long distances, making them effective in alerting nearby craft or coastal authorities.

Other options like single flashes of light and standard operational sounds do not explicitly indicate distress in the same manner. While single flashes of light can serve as a signal, they are more commonly used in specific communication contexts rather than as a universal distress signal. Calm navigation practices or standard operational sounds do not convey any request for help or indicate a situation of distress and are part of normal operations rather than signaling an emergency. Thus, the continuous horn blasts stand out as the clear option for a recognized distress signal.