This is one of those really annoying pure idiom questions. The correct idiom is "at temperatures as low as."

A. temperatures as low
correct!

B. temperatures so low
This structure can be correct in other contexts: "so low THAT something else happened." We can't say "so low AS"

C. as low temperatures
Within the structure "as _______ as," we have to use an adjective in the middle: as funny as, as old as, etc. Here, "low" is modifying "temperatures," so we have a noun in the structure. Incorrect.

D. as few
This one is particularly frustrating! We use "few" for countable items, so you'd think that we could say "few" with "degrees." But whenever we tie it to a specific measurement (-30 degrees), that's a single reading, not countable items. We say "these dogs weigh as much as 50 pounds," not "as many as 50 pounds." Ditto with "few" v. "little."

E. as little
... and this one is even more frustrating. In my previous example, we could have said "as little as 40 pounds." That's because what is implied with weight is that it's quantifiable: "a lot of weight" or "a little bit of weight." Temperature is different - you don't have "a lot" or "a little" of temperature. Temperature is not a quantity that exists in any amount. It's a measure on a scale that can be either high or low.

Does that help?