What is the volume of a cardboard box with sides a, b, and c ?

Based on what I've seen in my studying, though, we are not supposed to assume ANYTHING about any problem. In this case, I was unsure if I'm supposed to assume that a, b, and c are different sides, or if the answer is (E) because we don't know for sure that a and b aren't both the length of the box, just on different sides.

For a 2-d example, if the problem said there was a rectangle with sides a and b and asked me to calculate the area, I wouldn't assume that a and b are adjacent and not opposite sides unless the problem specifically told me that.

I know I must sound nitpicky but my exam is in 10 days and I've really gotten into the habit of not assuming a single thing that isn't given to us.

Valid concern. Come to think of it, if a question says
'a, b and c are sides of a rectangular box. What is the volume of the box?' I would not like to assume that a, b and c are length, breadth and height. They could be length, length and height. If they mention that a, b and c are distinct, then it is fine. (It is not essential that length, breadth and height of a box have to be distinct but if three sides are distinct, they have to be length, breadth and height.)
You do not assume anything in DS questions except for what is obvious (e.g. if in a group of people, there are 20% males, then the rest 80% are definitely female). It is sometimes a matter of judgment. Though, rest assured GMAT religiously follows its statistics i.e. if many 700 scorers are answering a particular 500 level question incorrectly, the question is deemed unfit and removed. So there is little chance that you will face even a single question that is debatable in actual GMAT.