In The Martian, the NASA mission that lands on Mars is Ares III.
Ares III is a fictional crewed NASA expedition to Mars in the 2015 film directed by Ridley Scott and adapted from Andy Weir’s novel. The mission’s crew spends 31 sols exploring Acidalia Planitia, a broad Martian plain. On the 18th sol, a severe dust storm forces an emergency evacuation. Astronaut Mark Watney is struck by debris and presumed dead, leaving him stranded after the rest of the crew departs aboard the Hermes.
Watney survives inside the mission’s surface habitat, or Hab, and uses his training as a botanist and engineer to extend his supplies. He grows potatoes, produces water from leftover rocket fuel, repairs communications, and eventually travels across Mars toward the landing site prepared for the later Ares IV mission.
Ares IV is therefore an easy answer to confuse with Ares III: it is the next mission, whose Mars Ascent Vehicle Watney plans to use for rescue. The story also mentions Ares II, but that mission belongs to the fictional sequence of earlier Mars expeditions. The Ares III crew ultimately alters the Hermes’ trajectory to return for Watney.