Well, it is asking you to compare Sarah's limping to something.
There are many possible answers, just think of things that limp.
Sarah limped along the road like an injured dog.
Sarah limped along the road like someone with a broken foot.
Sarah limped along the road like an elderly woman.
We have to do our homework before dinner, then we're free to watch TV, or go outside.
A—
This sentence pretty much sums up what the writer of the passage wants to talk about. The other ones do not fit with the sentence “water is one factor that is necessary for growth” and do not fit the idea of the passage, which is what factors affect plant growth.
