What class of medication primarily increases the levels of serotonin in the brain and is typically used to treat depression?

Antidepressants are specifically designed to increase the levels of serotonin in the brain, which plays a crucial role in mood regulation. This class of medication often includes selective serotonin reuptake inhibitors (SSRIs) like fluoxetine and sertraline, which work by preventing the reabsorption of serotonin into neurons, thereby increasing the availability of serotonin in the synaptic space. This mechanism helps alleviate symptoms of depression and is why antidepressants are primarily prescribed for this condition.

The other classes of medications listed do not primarily focus on increasing serotonin levels. Antipsychotics primarily target dopamine receptors and are generally used to treat conditions like schizophrenia. Stimulants are often used to treat attention deficit hyperactivity disorder (ADHD) and work by increasing levels of dopamine and norepinephrine rather than serotonin. Opioids are primarily pain-relieving medications that have a different mechanism of action and do not address mood disorders directly. Thus, antidepressants stand out as the correct answer for the treatment of depression through the modulation of serotonin levels.