ANS:B - partial pressure of the vapour to the vapour pressure of the liquid at gas temperature.

Relative humidity is the ratio of the partial pressure of the water vapor in the air to the vapor pressure of water at the same temperature. Explanation:

1. Partial Pressure of Water Vapor:
   • Relative humidity measures the amount of water vapor present in the air relative to the maximum amount of water vapor that the air could hold at a given temperature.
   • The partial pressure of water vapor represents the pressure exerted by the water vapor molecules in the air and is a component of the total atmospheric pressure.
   • As the temperature increases, the air can hold more water vapor, leading to higher partial pressure of water vapor.
2. Vapor Pressure of Water:
   • Vapor pressure is the pressure exerted by a vapor when it is in equilibrium with its condensed phase (liquid or solid) at a given temperature.
   • The vapor pressure of water is the pressure exerted by water vapor molecules when they are in equilibrium with liquid water at a specific temperature.
   • At higher temperatures, the vapor pressure of water increases, indicating that more water molecules can exist in the vapor phase at equilibrium.
3. Relative Humidity:
   • Relative humidity (RH) is expressed as a percentage and represents the ratio of the partial pressure of water vapor in the air to the vapor pressure of water at the same temperature.
   • Mathematically, relative humidity is calculated as:
   • Relative humidity indicates how close the air is to saturation with water vapor. A relative humidity of 100% means that the air is fully saturated with water vapor, while lower relative humidity values indicate that the air contains less water vapor relative to its capacity at that temperature.

In summary, relative humidity is the ratio of the partial pressure of water vapor in the air to the vapor pressure of water at the same temperature, representing the moisture content of the air relative to its saturation level. Therefore, the correct statement is: "partial pressure of the vapour to the vapour pressure of the liquid at the same temperature."