What is the main energy source for convection currents?

Correct answer: Earth's internal heat

The direct answer is B: Earth's internal heat. Convection needs energy to create differences in temperature and density. Inside Earth, heat from the deep interior warms parts of the mantle. Warmer material tends to become less dense and rise, while cooler, denser material sinks. This produces very slow convection within the mantle. Therefore Earth's internal heat is the main energy source named in this school-level question. Option A, external wind, moves air at the surface and is not the main source of mantle convection. Option C, river current, is surface water movement and cannot power circulation deep inside Earth. Option D, sea salt, can affect density in seawater but is not the principal energy source of mantle convection. A careful point is that actual mantle convection is complex and also involves gravity and cooling, but internal heat supplies the thermal energy that drives the process. Thus B is the intended and scientifically suitable answer. Memory cue: deep Earth heat makes mantle material rise and sink.