The primary cause of air moving towards the water from the land at night is related to the difference in temperature between the land and the water. Specifically, the ground cools faster at night than the water, which retains its heat longer.
As the land cools, the air above it also cools and becomes denser, leading to higher pressure over the land compared to the relatively warmer and less dense air over the water. This pressure difference causes the air to move from the land (high pressure) towards the water (low pressure), creating a breeze that is often referred to as a "land breeze."
So, the correct explanation from the options given is: The ground cools faster at night than the water, which retains its heat, warming the air above it.