Final answer:
The synaptic knob takes up acetylcholine after acetylcholinesterase breaks it down into acetyl and choline in the synaptic cleft, ending the neurotransmission and preventing unwanted prolonged muscle contraction.
Step-by-step explanation:
After acetylcholinesterase (AChE) acts, the synaptic knob takes up acetylcholine. AChE is an enzyme that resides in the synaptic cleft and breaks down acetylcholine (ACh) into acetyl and choline. This process prevents acetylcholine from remaining bound to the postsynaptic receptors, which would otherwise lead to prolonged neurotransmission and potential unwanted muscle contraction. Once ACh is degraded, the remaining components (acetyl and choline) can be recycled or taken back up by the presynaptic neuron. Therefore, the correct answer is (b) Takes up acetylcholine, as this indicates the presynaptic uptake of neurotransmitter components after the action of AChE.