What statement is true concerning heat transfer rates by the heat transfer mode of natural convection?

Correct Answer: Option A — Heat transfer rates are directly proportional to the difference in fluid densities which, in turn, is directly proportional to the temperature gradient.

Natural convection relies on buoyancy forces created by density differences in a fluid. As a fluid is heated, it expands and becomes less dense than the surrounding cooler fluid. This density gradient drives the circulation, meaning higher temperature gradients result in greater density differences and increased heat transfer rates.