Hi
The idiom
so as to is considered awkward. See the GMAT Official Guide (#65). In other words, expressions such as this are overly wordy. Always a good idea to look at the official material
so as to be able to determine what is proper usage.
However, in terms of usage, this idiom is not actually incorrect (the official guide even uses it in its explanations!). What usually happens is that these idioms are not incorrect but you can probably say the same thing in fewer words OR oftentimes this idiom wrong not because of the construction of the idiom but because it expresses what it wants to express wrongly or the sentence is simply not clear.
For example: The man wished to divorce his wife so as to go on a trip
Although it would seem clear that the man will go on a trip, it is not 100% clear who will go on a trip.
Therefore, look for other mistakes to eliminate the answer choice before you deal with the idiom, going from grammar mistakes first to stylistic errors (unless you are sure the idiom is wrong).