What type of signal should a vessel use to attract attention during the day?

Using visual signals such as flags is an appropriate method for a vessel to attract attention during the day due to their visibility and distinct nature. Flags can be easily seen from a distance and can convey messages quickly to other vessels or people on land. They are a standard means of communication in maritime operations, particularly in daylight when visibility is good.

While sound signals, like horns or whistles, are effective for signaling in certain contexts, they do not convey the same immediate visibility as visual signals during daylight hours. Flares, although effective for attracting attention, are designed primarily for emergency situations and are more pertinent for use at night or in low visibility conditions. Smoke signals can be utilized as well, but they may not be as reliable in every situation and can be affected by environmental factors like wind.

Visual flags, therefore, are specifically suitable for attracting attention during the day, making them the best choice among the options provided.