Nicotinic receptors a. are found at neuroeffector junctions of the parasympathetic nervous system. b. respond to epinephrine. c. open chemically-gated sodium ion channels. d. respond to norepinephrine. e. are found at neuroeffector junctions of the sympathetic nervous system.

Option C, open chemically gated sodium ion channels.

Explanation:

The nicotinic receptors are present on the muscle cell membrane. These receptors belong to the class of cholinergic receptor which binds to the acetylcholine and opens the gated channel to allow transfusion of cation through it. These actions of the receptors are stimulated by the neurotransmitter. Nicotinic receptors are very fast at acting and hence are more prevalent at the neuromuscular junction.

Hence, option  C is correct