What action is taken to adjust the handbrake on the specified vehicles?

The correct action to adjust the handbrake on specified vehicles involves turning the knob on the handbrake lever. This knob is designed to allow for fine-tuning of the brake system's tension, ensuring that the handbrake can correctly engage and hold the vehicle securely when applied. Proper adjustment is crucial, particularly in vehicles like the HMMWV, where reliable braking performance is paramount for safety and operational effectiveness.

The other options may refer to actions related to the operation or use of the handbrake rather than adjustment specifics. For example, pulling the handbrake lever engages or disengages the brake but does not adjust its functionality. Turning a screw under the dashboard or adjusting cable tension could be valid methods for different vehicle types or systems, but they are not the correct procedure for handbrake adjustment as specified in this context.