Final answer:
Substance P is the neuropeptide among the options given, which distinguishes it from other neurotransmitters like aspartate, acetylcholine, and glycine.
Step-by-step explanation:
The neurotransmitter in question that is a neuropeptide is substance P. Neuropeptides are neurotransmitter molecules composed of chains of amino acids connected by peptide bonds, shorter than typical proteins but still significant in their role in the body. Substance P is well-known within this family for its role as a neurotransmitter and neuromodulator in various nervous systems. It differs from amino acid neurotransmitters like aspartate, glycine, and neurotransmitters such as dopamine, serotonin, and norepinephrine which are not neuropeptides.