Final answer:
Glutamate is the brain's primary neurotransmitter, serving as the main excitatory signal in the CNS and is widely used by neurons.
Step-by-step explanation:
The primary neurotransmitter in the brain is glutamate. This amino acid acts as the main excitatory neurotransmitter in the Central Nervous System (CNS) and is utilized by roughly half of the neurons in the brain. Glutamate works by binding to specific receptors, such as AMPA, NMDA, and Kainate ionotropic receptors, leading to depolarization of the postsynaptic cell. This neurotransmitter is contrasted by GABA, which is the main inhibitory neurotransmitter. Knowing its role and widespread use among excitatory neurons, it's clear that the correct answer to the question is d. Glutamate.