The correct answer is option 4: Brain. Alzheimer`s disease is a neurodegenerative disorder that primarily affects the brain. In this disease, abnormal protein deposits form in the brain, leading to the damage and death of nerve cells. This results in a gradual decline in cognitive abilities, including memory loss, impaired thinking, and changes in behavior and personality. The progression of Alzheimer`s disease can vary from person to person, but it typically leads to severe impairment in daily functioning. Although Alzheimer`s disease may indirectly impact other parts of the body due to its effects on cognition and behavior, the primary organ affected is the brain.