Final answer:
Neurotrophins are the substances released by postsynaptic structures that maintain presynaptic neuronal structure.
Step-by-step explanation:
The substances released by postsynaptic structures that help to 'maintain' the structure of presynaptic neurons are known as neurotrophins. While neurotransmitters, like dopamine, are involved in the transmission of signals between neurons across the synaptic gap, neurotrophins play a crucial role in the survival, development, and function of neurons. The answer to the student's question is (a) Neurotrophins.