The most appropriate answer would be d. Serotonin and norepinephrine
The mechanism that cause depression is not clear, but it is known that the patient has lower level of serotonine, norepinephine and dopamine. The primary drug used for depression is selective serotonine reuptake inhibitor(SSRI), so you could infer that serotonine level is lower in those patient. There is also serotonine norepinephrine reuptake inhibitor(SNRI) drug used for depression.