Answer: B: too old
Explanation: To find our answer, let's use the process of elimination:
C and D make no sense, because C is worded wrong, and D is saying the man is old, so walks the stairs. Logically, that makes no sense. So, it has to be A or B. Looking at A and B, A says the man is so old to walk up the stairs. This can be true, but looking at B, A is informal. So, using the process of elimination, our answer is B. too old