A. so as to warm up the rooms [ so X as to be Y is the correct form of the idiom]

B. and so would be able to warm up the rooms [ wrong usage of 'and'. 'AND' needs to connect two independent clauses. the second is not a clause at all]

C. to warm the rooms up [ warm up is the correct idiom]

D. so that they could warm up the rooms [ correct! note 'so that' functions as 'in order' or rather as giving a reason. 'so that is not always wrong in GMAT.
There are two ways of constructing "so that" sentences.

Type 1 : I stopped running so that you could catch up. (here "so that" is used as "in order to")
Type 2 : The line was so long that i could not find the end of it.

E. in order that they would warm up the rooms [ nothing wrong in the sentence but wordier than D]