What type of weather phenomenon is indicated by thick gray clouds and heavy rain?

The correct answer is indicated by thick gray clouds and heavy rain, which are characteristic of a stationary front. A stationary front occurs when a warm air mass and a cold air mass meet but neither is strong enough to displace the other. As a result, the air masses remain in place, leading to prolonged periods of overcast skies and significant precipitation. The thick gray clouds are typical of the moisture-rich air associated with the warm front, while the consistent rain is a result of the lift and cooling of air that takes place along the boundary of the two air masses.

This phenomenon often leads to sustained rainfall and can result in the formation of stratus or nimbostratus clouds, which are both thick, gray, and associated with prolonged rain. In contrast, a warm front typically brings lighter rain or showers, while a cold front can lead to more intense, but shorter-lived, showers. A warm air mass doesn't describe a frontal boundary at all, nor does it indicate specific weather conditions on its own.