Runners stretch their legs while they’re waiting for the race to start.
“Their” is, in this case, speaking of ownership. The legs belong to the runners.
“They’re” is a contraction of “they are”. “They are waiting.” No possession is indicated in them waiting for the race to begin.
It can’t be A, since something goes in a chronological way no matter if it’s linear or nonlinear. I think it’s D since it has exciting points and then some not that exciting. Not sure but hope I helped.
True, depending on how a pronoun is used, it's case can change.