which is a plan for the future of space exploration?\nbuild a space station\nastronauts visiting…

Brief Explanations:

Building a space station has already been achieved (e.g., the International Space Station). Astronauts orbiting Earth has also been done for a long - time. Building a space observatory is an ongoing and past activity. Sending astronauts to Mars is a future - oriented plan for space exploration as it has not yet been accomplished on a human - crewed basis.

Answer:

astronauts visiting Mars