What flag should be flown by a vessel engaged in diving operations?

A vessel engaged in diving operations is required to fly a rigid replica of the International Code flag "A." This flag, which is also known as the "Alpha" flag, signals to other vessels that there are divers in the water and that they should maintain a safe distance to avoid interfering with the diving operations or endangering the divers' safety.

The specific requirement for a rigid replica ensures that the flag is clearly visible and maintains its shape in various weather conditions, making it easily recognizable from a distance. This is crucial for safety on the water, as it communicates to nearby vessels the need for caution and vigilance.

In terms of the other options, the International Code flag "D" is not associated with diving operations; it typically denotes a vessel is engaged in diving but not specifically in the way the Alpha flag does. The national ensign flag represents a country's sovereignty and does not convey any specific safety information related to diving activities. The yellow flag with a black circle is not recognized in the context of maritime operations involving diving and therefore would not adequately signal the presence of divers below the surface.