POLAR ICE

Antarctica receives more solar radiation than does any other place on Earth, yet the temperatures are so cold and the ice cap is reflective, so that little polar ice melts during the summer; otherwise, the water levels of the oceans would rise 250 feet and engulf most of the world’s great cities.

B. is so reflective that little of the polar ice melts during the summer; were it to do so,
"it" is used for "polar ice"

We can ignore everything before "yet" because "yet" is one of seven conjunctions that will tie one indepedent clause to another. As a result all that comes after "yet" should be able to stand alone as a full sentence.

At its core, answer B tells us:

B) Temps are so cold and the ice cap is so reflective that little of the polar ice melts; were it to do so, something bad would happen.

To make sense of what "it" refers to, think about the story the author is trying to convey. She writes that because the ice cap is so reflective, only a little of the polar ice melts; if it were to do so, something bad would happen.

"if it were to do so" --> "if it were to melt" --> "if the polar ice were to melt"

"it" refers to "polar ice"