Pressure that is below atmospheric pressure is measured in:

Pressure that is below atmospheric pressure is typically measured in units that indicate a vacuum or pressure differential. Inches of mercury is a common method of measuring negative pressure or vacuum levels. This is because as pressure decreases below atmospheric levels, the height of mercury in a column also drops correspondingly, making it a suitable measure for determining how much lower the pressure is than atmospheric pressure.

Inches of water, while also used to measure low pressures, is more commonly associated with slight pressure differences rather than a definitive measure of vacuum compared to atmospheric pressure. PSI (pounds per square inch) and kilopascals are units that generally denote absolute pressure rather than measuring negative or vacuum pressures specifically.

In summary, inches of mercury is the most appropriate and widely accepted method for measuring pressures that fall below atmospheric levels, making it the correct choice for indicating pressure in vacuum conditions.