Which of the following diseases is considered as the most common...

Alzheimer's disease is a progressive neurologic disorder that causes the brain to shrink and brain cells to die. It is the most common cause of dementia, which is characterized by a continuous decline in thinking, behavioral, and social skills that affects a person's ability to function independently.

Parkinson's disease is primarily a movement disorder.

Gull's disease is related to hypothyroidism (myxedema).

Grave's disease is an autoimmune disorder that causes hyperthyroidism.

Answer: Alzheimer's disease