A

The phenomenon can be explained by the concept of temperature inversion in atmospheric science.
Step 1: During the day, the ground gets heated, causing the air directly above it to be warmer than the air above it at higher altitudes. Warm air is less dense and causes sound waves to refract upwards, leading to a decrease in the sound intensity reaching the man.
Step 2: At night, the ground cools, and a layer of cooler air rests above the warm air. This creates a temperature inversion which traps the sound waves, allowing them to travel more effectively and reach the listener.
Therefore, on a clear night, the man hears the sound to be louder because of the refraction caused by the temperature inversion.