The correct conclusion from the given options is:
- The amount of water vapor in the air is 90% of what the air can hold.
This statement is a direct representation of what relative humidity means: it indicates the current amount of water vapor in the air as a percentage of the maximum amount of water vapor the air can hold at that temperature. In this case, with a relative humidity of 90%, the air is holding 90% of the water vapor capacity at 70 degrees Fahrenheit.