Question 309General Subjects - Assistant Engineer

The correct answer is A) Heat transfer rates are directly proportional to the difference in fluid densities which, in turn, is directly proportional to the temperature gradient. This is correct because in natural convection, the driving force for heat transfer is the difference in fluid densities caused by temperature differences. As the temperature gradient increases, the difference in fluid densities also increases, which in turn, directly increases the heat transfer rate by natural convection. The other options are incorrect because they do not accurately describe the relationship between heat transfer rates, fluid density differences, and temperature gradients in natural convection.