Rotating the __________ to zero engages altitude hold mode.

Rotating the vertical speed command wheel to zero engages altitude hold mode in the C-17. This action is a crucial component of the aircraft's autopilot systems, allowing the pilot to maintain the current altitude without needing to continuously manage the aircraft's vertical speed.

When the vertical speed command wheel is set to zero, the autopilot system captures the current altitude, preventing any further ascent or descent until the setting is changed. This is particularly useful during cruise phases of flight or when the aircraft is leveling off after climb or descent, as it allows the pilot to focus on other flight tasks without losing altitude.

The other choices like the altitude SEL knob, speed SEL knob, and heading SEL knob are related to different functions in the autopilot or flight management system, but they do not engage altitude hold mode directly. Each of these knobs influences different parameters—altitude settings, airspeed control, and heading navigation—yet do not serve the specific purpose of capturing and maintaining the flight altitude as the vertical speed command wheel does.