Final answer:
B) Acetylcholinesterase breaks it down. Acetylcholine (ACh) is broken down by the enzyme acetylcholinesterase (AChE) into acetyl and choline in order to prevent extended muscle contraction.
Step-by-step explanation:
ACh is broken down by the enzyme acetylcholinesterase (AChE) into acetyl and choline. AChE resides in the synaptic cleft, breaking down ACh so that it does not remain bound to ACh receptors, which would cause unwanted extended muscle contraction.