What is necessary for a convection cell to be set up in a fluid?

A) Changes in density

B) Thermal gradient

C) Thermal gradient and changes in the density

Convection cells don't work in solids.

In order for convection to happen, material with some temperature and density has to FLOW to a place where the material there has a different temperature and density.

Material doesn't FLOW in solids, only in FLuids.

Option (C)

Explanation:

In order to generate convection cells, the fluid must be of extremely high temperature and low density.

The lower part of the fluid when heated, the temperature increases significantly, as a result of which it expands. Due to this, it becomes less dense, and tends to rise up towards the surface. This rising fluid when reaches the surface, its temperature eventually decreases. It then becomes denser and subsequently sinks to the bottom. This is how the cycle repeats, forming the convection cells in the fluid.

Thus, both the thermal gradient as well as density is responsible for the formation of convection cells in a fluid.

Hence, the correct answer is option (C).