The part of the CNS that is responsible for these functions is the brain stem.
The brain stem is the part of the brain that control very basic functions of your organism and lets them work together as they should. The other parts that are mentioned all have much more specific and also different roles.
C. because stimuli is how your body responds to something, and in this case, someone's heart rate increases WHEN their blood oxygen drop. Do you see how it responds? ;-)
I think it's A....hope that helps at least
