The surface tension of liquid decreases with rise of temperature. The surface tension of liquid is zero at its boiling point and it vanishes at critical temperature. At critical temperature intermolecular forces for liquid and gases becomes equal and liquid can expand without any restriction. For small temperature differences, the variation in surface tension with temperature is linear and is given by relation

Explanation:
When the temperature of a liquid increases, the average kinetic energy of its molecules also increases. This increased kinetic energy causes the molecules to move more rapidly and increases the tendency for them to escape from the liquid's surface. As a result, the surface tension of most liquids decreases with an increase in temperature.

Surface Tension:
Surface tension is the force exerted by the surface of a liquid that tends to minimize its surface area. It is caused by the cohesive forces between the molecules in the liquid. These cohesive forces are responsible for the formation of a liquid drop and the formation of a meniscus in a capillary tube.

Effect of Temperature on Surface Tension:
The effect of temperature on surface tension depends on the nature of the liquid. In general, the surface tension of most liquids decreases with an increase in temperature. This can be explained by considering the molecular interactions within the liquid.

Explanation for the Correct Answer:
The correct answer is option 'C' - the surface tension of most liquids decreases with an increase in temperature. This is because as the temperature increases, the average kinetic energy of the liquid molecules increases. This increased kinetic energy disrupts the cohesive forces between the molecules, reducing the surface tension.

The exception mentioned in the question for molten copper and cadmium is due to their unique atomic structure and metallic bonding. In these metals, the surface tension actually increases with an increase in temperature. This is because as the temperature increases, the atoms in the metal become more mobile and can rearrange themselves into a more ordered structure, increasing the surface tension.

In summary, with an increase in temperature, the surface tension of most liquids decreases. However, molten copper and cadmium are exceptions to this general trend and exhibit an increase in surface tension with an increase in temperature.