The phenomenon of "trade winds" is primarily caused by option-2, convection of heat. Trade winds are the prevailing wind patterns found in the Earth`s tropics, blowing from the northeast in the Northern Hemisphere and from the southeast in the Southern Hemisphere. These winds are a result of the unequal heating of the Earth`s surface by the Sun.

Convection is the transfer of heat energy through the movement of fluids (in this case, air). In the tropics, the Sun`s radiation heats the Earth`s surface, causing the air above to warm and rise. As the warm air rises, it creates an area of low pressure near the surface. To fill this void, air from higher latitudes rushes in, creating a steady flow of wind known as the trade winds.

Option-1, conduction of heat, is the transfer of heat through direct contact between objects. While conduction occurs within the Earth`s surface, it is not the primary cause of trade winds.

Option-3, radiation, refers to the transfer of heat through electromagnetic waves. While radiation is responsible for heating the Earth`s surface initially, it does not directly cause trade winds.

Option-4, None of these, is not the correct answer as option-2, convection of heat, is