What action should be taken to open the valve allowing a higher volume of bleed air into the heating system?

Placing the DEFOG switch to ON is the appropriate action to allow a higher volume of bleed air into the heating system. The DEFOG switch specifically activates the system that directs bleed air to heat the canopy and prevent fogging on the windshield, which is critical for maintaining visibility in flight. When this switch is engaged, it ensures that warm air from the bleed air system is efficiently routed into the defogging ducts, enhancing the cockpit environment and comfort.

The other options, while they may relate to cabin management or air circulation, do not specifically operate to increase the volume of bleed air for heating purposes in the way the DEFOG switch does. For instance, simply turning on the DEFROST switch would not necessarily achieve the same result as the DEFOG switch since it serves a slightly different operational goal focused on improving visibility rather than actively warming the cockpit. Similarly, activating the HEAT switch generally facilitates the heating system but does not target the fogging issue as specifically as the DEFOG function. The FRESH AIR valve relates to the introduction of outside air but does not increase the bleed air for heating, making it less relevant for the specific need of defogging the cockpit.