Transfer of heat by convection depends upon:

Convection is the process of heat transfer through a fluid (liquid or gas) due to the movement of the fluid itself. The correct answer highlights that convection primarily occurs when a less dense fluid is displaced by a denser fluid. In this process, when a fluid is heated, it typically becomes less dense and rises. As it rises, it creates a region of lower pressure beneath it, allowing the denser, cooler fluid to flow in and take its place. This circulation pattern is essential for efficient heat transfer in fluids.

For example, in a heated pot of water, the water at the bottom warms up, becomes less dense, rises, and is replaced by cooler, denser water from above. This movement facilitates heat distribution throughout the fluid and enhances the overall heat transfer process.

The other choices relate to aspects of fluid characteristics, but they do not directly describe the fundamental mechanism driving convection in this context. The molecular structure of the fluid can influence properties like viscosity, and viscosity can affect fluid movement, but it is the difference in density that primarily governs the convective flow. Hence, the displacement dynamics between a less dense and a denser fluid are what facilitate the essential circulation needed for convection to occur.