Which of the following indicates that air is holding a maximum amount of moisture?

The correct understanding focuses on the concept of "saturation mixing ratio," which refers to the maximum amount of moisture that a given volume of air can hold at a specific temperature. When the saturation mixing ratio is high, it indicates that the air is at or near its maximum capacity for moisture. This means the air is fully saturated with water vapor, unable to hold any more without condensation occurring.

In contrast, high relative humidity, while it might indicate a significant amount of moisture in the air, does not necessarily equate to the air holding the maximum moisture unless it is at 100% saturation. Similarly, low dew-point temperature suggests less moisture in the air, and low relative humidity clearly indicates that the air contains a smaller proportion of water vapor compared to its potential maximum. Therefore, high saturation mixing ratio is the most accurate measure of the air being at its maximum moisture-holding capacity.