Acetylcholine is the most abundant neurotransmitter (molecule that sends signals from 1 nerve cell to another) in the body. One major function of acetylcholine is found at neuromuscular junctions. Here is transmits signal from a neuron to a muscle cell and is therefore responsible for initiating muscle contraction.
Not quite sure if you're given any answer choices, but here are some diseases that have been linked to having a deficiency with this neurotransmitter:
Parkinson's
Huntington
Alzheimer's
Myasthenia gravis (characterized by muscle weakness and fatigue)
