Final answer:
The correct option is b) so as not to wake.
Step-by-step explanation:
The correct option for the given sentence is b) so as not to wake.
When we use 'so as not to' in a sentence, it means that we do something in order to avoid a specific outcome or result. In this case, the person is opening the door quietly in order to avoid waking someone up. The phrase 'so as not to' is a common structure used to express purpose or intention.
Example: I turned off the TV so as not to disturb my sleeping baby.