The correct answer is option 3: the troposphere is heated by the Earth`s surface. The troposphere is the lowest layer of the atmosphere, extending from the Earth`s surface up to about 8-15 kilometers. It is predominantly heated by the transfer of heat from the Earth`s surface. The surface of the Earth absorbs solar radiation and then re-emits it as heat. This heat warms the air in the troposphere by conduction and convection. The air near the Earth`s surface is heated more than the air at higher altitudes, creating a temperature gradient. As a result, the troposphere is the hottest part of the atmosphere. Option 1, being closest to the Sun, is not the reason for the troposphere`s temperature. Option 2, the presence of charged particles, and option 4, heat generation, are not accurate explanations for why the troposphere is the hottest part of the atmosphere.