Radiation is the only method of transferring heat from the sun to the Earth. Conduction requires a solid substance. For example, if you hold one end of a metal pipe and put the other end in a fire, the heat moves from the fire to your hand by conduction through the metal. The heat is basically being transferred from one molecule to the next along the solid. Convection requires a fluid (liquid or gas) to conduct heat. Convention basically means that molecules of the gas or liquid become hot and then move through the fluid by currents (especially hotter fluids rising like hot air above a radiator) or diffusion, carrying the heat with them.