Answer:
b.a heat source
Explanation:
Convection occurs in fluids. This phenomenon occurs when a fluid is heated: a classical example is the pot containing water over a flame. As the water is heated by the flame, the bottom part of the water becomes warmer than the upper part: as a result, the bottom part of the water goes up, replacing the colder part which goes down. Then, this colder part is heated, and it goes up again, etc. in a cycle. This phenomenon cannot occur if there is no heat source that heats the fluid.
