**Core Concept**
Alzheimer's disease is a neurodegenerative disorder characterized by the accumulation of amyloid-beta plaques and tau protein tangles in the brain, leading to neuronal loss and cognitive decline. The treatment of Alzheimer's disease focuses on managing symptoms, slowing disease progression, and improving quality of life.

**Why the Correct Answer is Right**
Donepezil, a cholinesterase inhibitor, is used to treat Alzheimer's disease by increasing the concentration of acetylcholine in the brain. Acetylcholine is a neurotransmitter that plays a crucial role in memory and cognitive function. By inhibiting the enzyme acetylcholinesterase, donepezil prevents the breakdown of acetylcholine, thereby enhancing cholinergic transmission and alleviating symptoms of Alzheimer's disease. This mechanism is particularly beneficial in patients with mild to moderate Alzheimer's disease.

**Why Each Wrong Option is Incorrect**
**Option A:** Tacrolimus is an immunosuppressant used to prevent organ rejection in transplant patients, and it is not indicated for the treatment of Alzheimer's disease.
**Option B:** Valproate is an anticonvulsant medication primarily used to treat epilepsy and bipolar disorder, and it is not a standard treatment for Alzheimer's disease.
**Option C:** Metformin is an oral antidiabetic medication used to treat type 2 diabetes, and it is not used for the treatment of Alzheimer's disease.

**Clinical Pearl / High-Yield Fact**
Cholinesterase inhibitors like donepezil are most effective in patients with mild to moderate Alzheimer's disease, and their use should be carefully monitored for potential side effects such as nausea, vomiting, and diarrhea.

**Correct Answer:** C. Donepezil.