Hello!

Let's break down each answer for its individual problems.

A: Correct answer; no question of what -ing phrase is referring to
B: By saying "to uncover," it suggests that the ice cap somehow chose to melt because it wanted to uncover land. I know it sounds nutty to think about it that way, but the way it's worded makes it sound like the ice cap had a choice in the matter.
C, D, and E: By putting the words "by melting," "after melting," or "through melting" between two commas, it turns each into a non-essential phrase that could be removed from the sentence. Unfortunately, if we take out the part about the ice cap melting, we now don't know how the ice cap created new lands for people to live in. Did it move? Did it melt? Did it fly away? We don't know.

So answer A is correct because the modifier is located next to what it's referring to without changing or confusing the meaning of the sentence.

I hope that helps. I'm available if you'd like any follow up.