The correct answer is "depression".
Selective serotonin reuptake inhibitors or SSRI is the one responsible for the mainstay of treatment of major depressive episodes or major depressive disorder. The pathology in depression is the low levels of serotonin, that's why an SSRI inhibits the reuptake of serotonin in the synapses making its levels high therefore inhibiting the signs and symptoms of depression. Examples of SSRI is Fluoxetine and Escitalopram.