Migraine headaches are most closely linked with an

Glutamate

Explanation:

One promising area of research on new migraine treatments involves the chemical glutamate, one of the main neurotransmitters in the brain. A substance released by one nerve cell that allows it to communicate with another is a neurotransmitter. This communication or interaction occurs through a structure on the surface of the cell (or inside the cell), known as a receptor. Receptors selectively receive and lock onto specific substances, such as glutamate. Glutamate functions as an “excitatory” neurotransmitter, because when released it causes neighboring cells to become more active (excited).

Increasing evidence supports that excessive activation of cells in specific areas of the brain causes migraine. Special brain scans show that patients having migraine attacks may have waves of abnormal activity that spread across the surface of the brain (the cortex), as well as excitation of nerve centers deep within the brain (the brainstem). Both of these patterns of abnormal activity may involve the neurotransmitter glutamate.

d would be your answer