Question

which is a plan for the future of space exploration? build a space station astronauts visiting mars astronauts orbiting earth build a space observatory

Explanation:

Building a space station, astronauts orbiting Earth, and building a space observatory are already ongoing or have been achieved to some extent. Astronauts visiting Mars is a future - oriented plan in space exploration as it has not yet been fully realized on a large - scale human mission basis.

Answer:

astronauts visiting Mars