Final answer:
Otto Loewi's experiments showed that the presynaptic neuron releases acetylcholine, which stimulates the postsynaptic neuron.
Step-by-step explanation:
Otto Loewi's experiments demonstrated the role of neurotransmitters in synaptic transmission. Acetylcholine is a neurotransmitter that, when released from the presynaptic neuron, binds to receptors on the postsynaptic membrane and can have excitatory or inhibitory effects on the postsynaptic neuron. In Loewi's classic experiment, the release of acetylcholine resulted in the depolarization of the postsynaptic membrane at the neuromuscular junction, showing an excitatory effect that increases the likelihood of an action potential in the postsynaptic neuron. Therefore, the answer to the student's question is: a) Presynaptic neuron; stimulates the postsynaptic neuron.