ANS:B - density

Heat transfer occurs by natural convection because a change in temperature causes a difference in density. When a fluid is heated, it expands and becomes less dense, leading to buoyancy forces that cause the fluid to rise. Conversely, when a fluid is cooled, it contracts and becomes more dense, causing it to sink. This movement of fluid due to density differences is known as natural convection, and it facilitates heat transfer by carrying heat away from hotter regions and distributing it to cooler regions within the fluid. Therefore, the correct answer is: density