The correct answer is: Dew point temperature.
Dew point temperature is the temperature at which the air is saturated with water vapor and condensation begins. It is the best indicator of the actual amount of water vapor in the air because it is directly related to the amount of water vapor that the air can hold.
Air temperature is not a good indicator of the actual amount of water vapor in the air because it can vary greatly depending on the amount of moisture in the air. For example, the air temperature can be very high in a desert, but the relative humidity can be very low, meaning that there is very little water vapor in the air.
Saturation vapor pressure is the pressure of water vapor in a saturated atmosphere. It is a measure of the maximum amount of water vapor that the air can hold at a given temperature. Saturation vapor pressure increases with increasing temperature.
Relative humidity is the ratio of the amount of water vapor in the air to the amount of water vapor that the air can hold at a given temperature. It is expressed as a percentage. Relative humidity is a good indicator of how comfortable the air feels. For example, if the relative humidity is 100%, the air is saturated and will feel very humid.
In conclusion, dew point temperature is the best indicator of the actual amount of water vapor in the air because it is directly related to the amount of water vapor that the air can hold.