Building a space station, in which astronauts would live for a conside

Let's start by breaking down the passage.

The argument concludes that "Building a space station, in which astronauts would live for a considerable time, is essential..." But why is it essential? Well, because for future missions to explore Mars, "we will need the medical knowledge that the space station project will give us about the limits of human capacities to live in spacecraft for an extended time."

As you say, an assumption should be something not explicitly stated in the passage. It should also be necessary for the argument to hold. Let's take a look at (A) to see if it fits the bill:


So how does this relate to the passage?

Notice the argument never explicitly tells us that the exploration of Mars will be carried out by people. It does tell us "astronauts would live for a considerable amount of time" in the space station, but the space station itself won't be used to explore Mars. Rather the space station will give us "medical knowledge" about "the limits of human capacities to live in spacecraft for an extended time."

So (A) is not explicitly stated in the passage. But is it necessary?

Well, if we're just going to send robots to Mars, we wouldn't actually need any "medical knowledge" about "human capacities to live in spacecraft." In other words, to justify the the conclusion that the space station is essential, we have to assume that "the exploration of Mars will be carried out by people traveling in spacecraft and not by robots alone."

Since (A) is not explicitly stated, and it's necessary for the argument to hold, it's the correct answer.

I hope that helps!