Use the _____ to control the VHF NAV volume.

The NAV knob on the audio control panel is specifically designed to adjust the volume of the VHF navigation radio in the T-6 aircraft. This control allows the pilot to set the volume to a comfortable level to ensure that navigation communications can be heard clearly without interference from other audio sources.

In the context of aircraft systems, the audio control panel serves as the central hub for managing various audio inputs and outputs, including VHF NAV communications. Adjusting the volume through the NAV knob is crucial for maintaining situational awareness and effectively receiving navigation information.

The other options do not provide the same functionality for controlling VHF NAV volume. The main control panel does not typically include specific volume controls for navigation radios, while the VHF frequency monitor displays frequencies but does not adjust volume levels. The volume slider on the radio itself might control audio output for different frequencies or channels, but it is not the designated method for the NAV radio specifically.