Answer:
Option C, open chemically gated sodium ion channels.
Step-by-step 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