Which is required for convection to occur?

a. a solid substance
b. a heat source
c. boiling water
d. air molecules

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.

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