Fog that most often forms as warm rain falls into a cold layer of surface air is called:

Radiation fog
Evaporation (mixing) fog
Advection fog
Upslope fog

The correct answer is: C. Advection fog

Advection fog is a type of fog that forms when warm, moist air moves over a cold surface. The warm air cools as it comes into contact with the cold surface, and the water vapor in the air condenses to form fog. Advection fog is most common in coastal areas, where the warm, moist air from the ocean moves over the cold land. It can also form in inland areas, when warm, moist air moves over a cold lake or river.

Radiation fog is a type of fog that forms when the ground cools rapidly at night. As the ground cools, it cools the air above it. The cool air can no longer hold as much water vapor, and the water vapor condenses to form fog. Radiation fog is most common in clear, calm nights, when there is little wind to mix the air.

Evaporation (mixing) fog is a type of fog that forms when warm, moist air mixes with cool, dry air. The warm air rises, and as it rises, it cools. The cool air can no longer hold as much water vapor, and the water vapor condenses to form fog. Evaporation fog is most common in areas with a lot of water vapor, such as near rivers or lakes.

Upslope fog is a type of fog that forms when moist air moves up a hill or mountain. As the air moves up, it cools. The cool air can no longer hold as much water vapor, and the water vapor condenses to form fog. Upslope fog is most common in mountainous areas.