In a psychrometric chart, one axis represents dry-bulb temperature. The axis that is perpendicular to the dry-bulb temperature axis represents:

The correct option is specific humidity.

To understand why this is correct, let's analyze the layout of a standard psychrometric chart:
A psychrometric chart is a graphical representation of the thermodynamic properties of moist air. It is plotted with two main coordinate axes at a right angle (perpendicular to each other):

1. Abscissa (Horizontal Axis / X-axis): This axis represents the dry-bulb temperature (usually in °C or °F), with temperature values increasing from left to right.

2. Ordinate (Vertical Axis / Y-axis): The axis that is perpendicular to the horizontal dry-bulb temperature axis is the vertical axis on the right side of the chart. This vertical axis represents the specific humidity (also referred to as moisture content or humidity ratio, typically expressed in grams of water vapor per kilogram of dry air, g/kg, or kg/kg).

While other properties like wet-bulb temperature, relative humidity, and enthalpy are also plotted on the psychrometric chart, their lines of constant values are oblique (diagonal) or curved curves across the chart, rather than being perpendicular to the dry-bulb temperature axis. Therefore, the axis directly perpendicular to the dry-bulb temperature axis is the specific humidity axis.