Final answer:
The answer to the question regarding larger molecules involved in slower, ongoing neural activity, as compared with small-molecule neurotransmitters, is neuropeptides (b.).
Step-by-step explanation:
The answer to the student's question: As compared with small-molecule neurotransmitters, larger ______ are involved in slower, ongoing neural activity is neuropeptides (b.). Unlike small-molecule neurotransmitters such as GABA, glutamate, and serotonin, which are involved in rapid signaling, neuropeptides often function in neuromodulation. This means they are involved in slower and more ongoing neural activity. Neuropeptides are larger molecules that include protein molecules and shorter chains of amino acids. They do not fit into the same systems as amino acid neurotransmitters like GABA or glutamate, which clear from the synaptic cleft by reuptake, nor do they have the simple structure of monoamines like dopamine or serotonin.