Final answer:
Michael Collins, the third member of Apollo 11, remained in the Command Module and was responsible for its operations, took scientific measurements, and photographed the moon, in preparation for the lunar module's return.
Step-by-step explanation:
While Neil Armstrong and Buzz Aldrin were walking on the moon, the third member of the Apollo 11 mission, Michael Collins, remained in lunar orbit aboard the Command Module, Columbia. His tasks were critical to the success of the mission. He was responsible for maintaining the command module's systems, taking scientific measurements, photographing the lunar surface, and preparing for the crucial task of rendezvousing and docking with the Lunar Module once Armstrong and Aldrin ascended from the moon's surface.
During the approximately 21 hours that his fellow astronauts were on the lunar surface, Collins orbited the moon, ensuring that he retained the ability to return to Earth even if something went wrong on the surface. Due to his isolated position, he is sometimes referred to as the 'loneliest man in history' during the time he was on the far side of the moon, completely out of contact with Earth.