Professor murphy is studying neurons that release a particular small-molecule neurotransmitter. in these many of these neurons, the professor should expect to see no other types of neurotransmitters. at least one type of neuropeptide neurotransmitter. at least one other type of small-molecule neurotransmitter and multiple types of neuropeptides neurotransmitters. a second type of small-molecule neurotransmitter.

I believe the answer is at least one type of neuropeptide neurotransmitter.  Neurotransmitters are the chemicals which allow the transmission of signals from one neuron to the next across synapses. They are also found at the axon endings of motor neurons, where they stimulate the muscle fibers. Neuropeptides are one type of neurotransmitters and they are composed  only of peptides with larger molecular weights, a key difference between neurotransmitters and neuropeptides.