The correct option is D Dopamine
Parkinson's disease is caused by degeneration of neurons in Substantia Nigra tract which are essentially dopaminergic.
Dopamine is required by the striatum to controls muscle tones and coordinates it movements.
Therefore, when dopamine levels are too low, people find it harder to get things done, to control their movements.
As dopamine levels progressively drop in patients with the disease, their symptoms gradually become more severe.