What is the first step in building a flight plan on the mission computer?

The first step in building a flight plan on the mission computer is selecting the ROUTE DATA function. This is because ROUTE DATA allows the pilot to set up the specific waypoints, airways, and other necessary elements that define the intended flight path. By inputting the route data first, the mission computer can accurately calculate distances, estimated times of arrival, and fuel requirements based on the selected route. Other options such as FLT PLAN, LAT REV AT, and VERT REV AT relate to different aspects of flight planning and management, but they follow the initial input of route data, which establishes the fundamental structure for the overall flight plan.