Question

unlike the short flights of the shuttle and earlier spacecraft, which carried sufficient power in fuel cells and batteries

Answer 1

Its a comparison question. We need to make sure after unlike we have the correct thing. I mean the one that can be compared to a permanently orbiting space station. So this is very clear that flights cannot be compared to a permanently orbiting space station. This eliminates option A,C, and E. Majority of the job done !!!  

And out of B and D the better choice is D because it uses WHICH, which properly describes the short flights of the shuttle and earlier spacecraft  

Answer 2

Explanation:

Unlike the short flights

earlier spacecraft.

• which carried sufficient power in fuel cells and batteries, a permanently orbiting space station will have to generate its own electricity.