The correct answer is E. Radiation.
Evaporation is the process by which water changes from a liquid to a gas or vapor. Reflection is the process by which light or other waves are thrown back from a surface. Conduction is the transfer of heat between two objects that are in direct contact with each other. Convection is the transfer of heat through the movement of fluids.
On a clear, calm, night, the ground and air above cool mainly by radiation. This is because the ground and air above are able to radiate heat into space without any obstructions. The heat from the ground and air above is absorbed by the atmosphere, which then radiates the heat back into space. This process of radiation is what causes the ground and air above to cool on a clear, calm, night.
Evaporation, reflection, conduction, and convection are all important processes in the transfer of heat, but they are not the main processes that cause the ground and air above to cool on a clear, calm, night.