Pressing the ___________ pushbutton engages altitude hold mode, maintaining the current altitude.

The engagement of altitude hold mode in the C-17 is achieved by pressing the ALT HOLD pushbutton. This function is designed to maintain the aircraft's current altitude automatically, allowing pilots to focus on other aspects of flight management. When activated, the altitude hold mode captures the aircraft's present altitude and prevents undesired climbs or descents, enabling smoother operations, especially during longer flights or when managing other systems and navigation tasks.

The other options, while relevant to aircraft operations, serve different functions. For instance, VNAV (Vertical Navigation) involves managing vertical trajectory using waypoints and is typically used for more complex flight profiles. The TURB button would relate to turbulence modes for stabilizing the flight, and SPLIT AXIS refers to control of aerodynamic surfaces rather than altitude maintenance. Thus, the ALT HOLD pushbutton is specifically designed for capturing and holding altitude, which is why it is the correct answer.