What flag is used during the day for a storm warning?

The correct flag used during the day for a storm warning is the red flag with a black square. This specific flag indicates dangerous weather conditions, particularly winds and waves that could be hazardous to vessels at sea or in port.

In maritime communications, flags serve a crucial role in conveying important information quickly. The design of the red flag with a black square is internationally recognized among mariners as a warning sign, which means that there is an imminent threat due to severe weather patterns or conditions that sailors need to heed for their safety.

The other choices do not represent the established signal for storm warnings. For example, while the two red flags with a black square have their own meanings in different contexts, they do not specifically signify a storm warning. The red pennant typically indicates a caution signal, and the white over red flag arrangement typically signifies a different maritime message. Thus, understanding the specific meanings attached to these flags is vital for safe navigation and communication on the water.