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.

A. is reflective, so that little polar ice melts during the summer; otherwise,
B. is so reflective that little of the polar ice melts during the summer; were it to do so,
C. so reflective that little polar ice melts during the summer, or else
D. reflective, so that little of the polar ice melts during the summer, or
E. reflects so that little of the polar ice melts during the summer; if it did

pLEASE PROVIDE EXPLAINATION

I think it is B. What is OA?

The answer is indeed B.

Only B and C have acceptable parallelism with the 'so' construction ('the temps are so cold and the ice cap [so] reflective...'), so it's down to those two.

C is badly worded, though, because it seems to imply that there are two possibilities of essentially equal probability (either (1) little polar ice melts OR ELSE (2) the water levels blah blah blah). That leaves B, which correctly implies that the melting of the ice is hypothetical.