Final answer:
Depression is commonly associated with low levels of both norepinephrine and serotonin, which are neurotransmitters thought to play a key role in mood regulation.
Step-by-step explanation:
In terms of neurotransmitter levels, depression is primarily associated with low norepinephrine and low serotonin levels, which corresponds with option D. The "classic monoamine hypothesis" suggests that a decrease in neurotransmission of both norepinephrine and serotonin contributes to the symptoms of depression. Studies and treatments for depression often target these neurotransmitters. Selective serotonin reuptake inhibitors (SSRIs), for example, block the reuptake of serotonin into the presynaptic neuron, increasing serotonin levels in the synaptic cleft and helping to alleviate depressive symptoms.