Explanation:

Nusselt number is a dimensionless number used in heat transfer calculations. It is defined as the ratio of convective to conductive heat transfer across a boundary. The value of Nusselt number depends on the flow conditions, geometry, and physical properties of the fluid.

The convective surface coefficient h is a measure of the rate of heat transfer from the surface to the fluid. It is directly proportional to the Nusselt number, which means that if the Nusselt number increases, the convective surface coefficient also increases.

The convective surface coefficient h depends on various physical properties of the fluid, such as thermal conductivity, density, viscosity, and specific heat. Out of these properties, thermal conductivity has a direct effect on the convective surface coefficient.

Thermal conductivity is a measure of the ability of a material to conduct heat. It is defined as the rate of heat transfer through a unit area of a material of unit thickness when a unit temperature gradient is maintained across the material.

The convective surface coefficient is directly proportional to the thermal conductivity of the fluid. This means that if the thermal conductivity of the fluid increases, the convective surface coefficient also increases.

Therefore, option C, thermal conductivity, is the correct answer.

In summary:

- Nusselt number is a dimensionless number used in heat transfer calculations.
- Convective surface coefficient h is a measure of the rate of heat transfer from the surface to the fluid.
- The convective surface coefficient h is directly proportional to the Nusselt number.
- Thermal conductivity is a measure of the ability of a material to conduct heat.
- The convective surface coefficient is directly proportional to the thermal conductivity of the fluid.
- Therefore, option C, thermal conductivity, is the correct answer.