Final answer:
The correct answer is Presynaptic dopaminergic depletion. Reserpine depletes presynaptic monoamine neurotransmitters, which can improve symptoms of dystonia but may also cause depression.
Step-by-step explanation:
The mechanism of action by which reserpine both improves the symptoms of adult-onset primary focal dystonia and can cause depression is Presynaptic dopaminergic depletion (Option D). Reserpine works by depleting the presynaptic stores of monoamines, such as dopamine, norepinephrine, and serotonin, thereby reducing their availability for release into the synaptic cleft. The depletion of dopamine may help with the symptoms of dystonia, which is associated with abnormal dopaminergic transmission. However, this same action can lead to a deficiency in monoamines that are critical for mood regulation, potentially causing depression.