Final answer:
A diver's flag is red with a white diagonal stripe, serving as a warning signal to others that a diver is present in the water.
Step-by-step explanation:
The appearance of a diver's flag is red with a white diagonal stripe. This flag is internationally recognized and signals to others that there is a diver below and they should keep a safe distance to avoid any accidents. The red and white diver's flag is different from a nautical flag or other banners, like the Flag of Chicago, which is not related to diving but instead symbolizes geographical features such as waterways.