Final answer:
Option (D), Antidepressant drugs mainly raise the level of serotonin in the brain. SSRIs work by preventing the reuptake of serotonin, thus increasing its availability.
Step-by-step explanation:
Antidepressant drugs primarily work by raising the level of a neurotransmitter in the brain. The correct answer to the question is (D) Serotonin. Selective serotonin reuptake inhibitors (SSRIs) are a class of antidepressant medications that block the reuptake of serotonin into the presynaptic neuron, thereby increasing its concentration in the synaptic cleft.
Serotonin is believed to play a key role in mood regulation, and low levels are associated with depression. It is important to note that other neurotransmitters such as dopamine and norepinephrine are also targets for different classes of antidepressants.