When it is impracticable for a small vessel engaged in diving operations to display shapes for underwater operations, it must display:

The correct answer is to display a rigid replica of the International Code flag "A." This flag signals that a vessel has divers working below the surface and serves to inform other water users to exercise caution and maintain a safe distance. When a vessel is unable to display the traditional shapes for underwater operations due to impractical circumstances, utilizing the flag becomes essential to ensure the safety of both the divers and nearby vessels.

This method of signaling is recognized internationally and emphasizes the importance of communication on the water, especially in situations involving divers. Displaying the flag is a clear and effective way to indicate that there are potential hazards below and to prevent accidents that could arise from other vessels failing to recognize the presence of diving operations.

The other options, while they may represent different navigational signals or shapes used in various contexts, do not specifically meet the requirement of signaling for diving operations as recognized by maritime regulations. Hence, they would not provide the same clarity or enforce safety standards in situations where divers are working underwater.