The correct answer is option 'B': The wet-bulb temperature is the same as the dew point temperature at 100% relative humidity.

Explanation:
Relative humidity is a measure of how much moisture is in the air compared to the maximum amount it can hold at a given temperature. It is expressed as a percentage. When the air is saturated with moisture and holding the maximum amount of water vapor it can at a particular temperature, the relative humidity is 100%.

The dew point temperature is the temperature at which the air becomes saturated and can no longer hold all the moisture it contains. When the air reaches its dew point temperature, condensation occurs, and water droplets form. The dew point temperature is a measure of how much moisture is in the air, regardless of the air's temperature.

The wet-bulb temperature is a measure of the lowest temperature that can be achieved by evaporating water into the air. It is measured by wrapping a wet cloth around the bulb of a thermometer and then swinging it through the air. Evaporation from the cloth cools the thermometer bulb, and the wet-bulb temperature is the lowest temperature recorded during this process.

At 100% relative humidity, the air is already saturated with moisture, and the dew point temperature has been reached. This means that the air cannot hold any more water vapor without condensation occurring. Therefore, when the wet-bulb temperature is measured under these conditions, it will be the same as the dew point temperature. This is because the air is already at its maximum capacity for holding moisture, and evaporation from the wet cloth on the thermometer will not result in any additional cooling.

In summary, at 100% relative humidity, the wet-bulb temperature is the same as the dew point temperature because the air is already saturated with moisture and cannot hold any more water vapor without condensation occurring.