Which is required for convection to occur?

A. a solid substance

B. a heat source

C. boiling water

D. air molecules

A heat source is the correct answer

Convection currents cannot travel through a solid, so A can't be the answer.

While boiling water would have convection currents running through it, it's not a prerequisite for them to occur. Similarly, air molecules can be part of a convection current but air''s not the only thing convection currents can occur in. This eliminates C & D.

Hence, B - a heat source - is the correct answer.