Step 1: Understanding the Concept
Parkinson's disease is a movement disorder with tremor, rigidity and slow movement.
Step 2: Cause
It arises from degeneration of dopamine-producing neurons in the substantia nigra of the midbrain, which lowers dopamine in the basal ganglia.
Step 3: Why the other options fail
Loss of cholinergic neurons and amyloid deposits are features of Alzheimer's disease. Under-activity of beta cells causes diabetes mellitus.
Final Answer:
It is caused by degeneration of dopamine-producing neurons.
\[ \boxed{\text{(D) }\text{degeneration of dopamine producing neurons.}} \]