I chose norepinephrine but the answer is acetylcholine. Why?
You could also have answered the question by asking, "Which one of these is not like the others?" Dopamine, norepinephrine, and epinephrine are all derived from tyrosine while acetylcholine is not. Dop., nor. and ep. are all catecholamines while ACh is not. Stimulant drugs increase the amount of transmission for dop, nor. and ep. but do not normally do so for ACh.
But more directly to the question: ACh slows the heart down, which is a prime example of parasympathetic nervous system activation.
I wonder if this has something do do with "primary" being pre-ganglionic neurons, in which case all autonomic preganglionic neurons have acetylcholine as their neurotransmitter. I thought where the sympathetic and parasympathetic differed was the postganglionic neuron. If I recall, sympathetic is NE and parasympathetic is acetylcholine. Maybe this is what was meant by primary?
This diagram will help: http://images.books24x7.com/bookimages/id_14427/fig17-1.jpg
As for the answer to why? Ask god lol