(A) is most tempting through the blurry lens of “doing all that nonsense isn’t practical,” which is indeed the argument’s broad thrust. But (A) says something much more specific: it says mining the Moon isn't practical right now.

If the argument’s whole purpose was to show that mining the Moon isn’t currently feasible, then all the discussion of fusion reactors and solving energy problems would be entirely irrelevant. The stimulus would instead feature a lot more discussion of why it’s not currently feasible.

This claim doesn’t even appear in the stimulus, so it’s certainly not the main point.

The stimulus tells us we’re 50 years away from helium-3 fusion reactors, but that doesn’t mean there are currently fusion reactors of other kinds.

(C) starts out great: we want “people who advocate mining the Moon for helium-3 [are talking some nonsense].” The second part is inaccurate, though – the author never talks about the possibility of using other fuels.

This matches our stimulus – it tracks the word “this” back to the argument “some people” are making about how we can solve the world’s energy problems by mining the Moon for helium-3, and says “No! That is nonsense!”

This is a premise that supports the argument’s main claim that mining the Moon is a dumb pitch for solving the world’s energy problems: “if we try it your way we’ll be too late, therefore your way is dumb.”

For help understanding the process you can use to see that (E) supports the main conclusion and not the other way around, see this lesson.