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.
Option Elimination -
(A) is reflective, so that little polar ice melts during the summer; otherwise, - The right idiom is so (cause), that (effect). Here, the idiom is messed up, and the meaning is not as intended.
(B) is so reflective that little of the polar ice melts during the summer; were it to do so, - perfect. Right idiom structure so (cause), that (effect). Moreover, this eloquently expresses the conditional. E.g., If I were rich, I would travel around the world (Expresses a hypothetical or unreal situation in the present. In reality, the speaker is not rich).
(C) so reflective that little polar ice melts during the summer, or else - We need "is" before "so" to complete the clauses in order for it to be parallel to the clause before "and" i.e. the temperatures are so cold. Moreover, or else messes up with the meaning as if either "little polar ice melts during the summer" or "the water levels of the oceans would rise 250 feet and engulf most of the world’s great cities."
(D) reflective, so that little of the polar ice melts during the summer, or - the same issue as C We need "is" + the idiom issue. Again the meaning is messed up.
(E) reflects so that little of the polar ice melts during the summer; if it did - the idiom issue, leads to the meaning issue.