(D) ...they will have to learn how to deal with this weird environment.
(A) The supporting details serve to expand upon the main idea, fully describing what life is like in a weightless environment.
Explanation:
The main idea of the selection is that even in a one-year trip, which is the shortest amount of time required to get to Mars, they will have to learn how to cope with the lack of gravity because its effects on the body are harsh.
The selected summary statement is the only one that correctly describes the selection as providing supporting details about how unpleasant weightlessness can be to support the idea of astronauts having to learn how to deal with it.
