The correct answer is option 4 - terrestrial radiation.

Cloudy nights are generally warmer than clear nights because clouds act as a blanket or barrier that traps heat radiating from the Earth`s surface. This is known as the greenhouse effect, which helps to keep the Earth warm.

Option 1 - greenhouse effect: This is mentioned in the explanation above and is correct. The greenhouse effect is the process by which greenhouse gases, such as carbon dioxide, trap heat in the Earth`s atmosphere.

Option 2 - depletion of ozone layer: The depletion of the ozone layer is not directly related to the temperature difference between cloudy and clear nights. Ozone depletion primarily affects the amount of harmful ultraviolet (UV) radiation that reaches the Earth`s surface.

Option 3 - insolation: Insolation refers to the amount of solar radiation that reaches the Earth`s surface. While insolation is an important factor in determining temperature, it is not the primary reason why cloudy nights are warmer than clear nights.

In conclusion, the correct explanation is that cloudy nights are warmer than clear nights because clouds trap heat radiating from the Earth`s surface, known as terrestrial radiation.