Which pennant indicates a need for communications with another vessel?

The correct choice indicating a need for communications with another vessel is the preparation pennant. This pennant serves as a visual signal to alert other vessels that you are ready to communicate or exchange information at that time. It is particularly important in maritime operations where clear communication is essential for safety and coordination, allowing vessels to prepare for actions such as maneuvering, deploying, or receiving instructions.

The other options denote different meanings within naval signaling. For example, the third substitute typically indicates that communication is in progress with a specific vessel but does not directly signal that communication is needed. The SOPA pennant is used to designate the senior officer present afloat and does not pertain directly to communication needs. The Alpha Flag, on the other hand, signals that a vessel is divers diving and requires attention; it indicates presence rather than a request for communication. Thus, the preparation pennant stands out as the proper visual indicator for initiating communication with another vessel.