Explanation:

- Scurvy is a disease resulting from a deficiency of vitamin C, which is also known as ascorbic acid. Symptoms include weakness, anemia, gum disease, and skin bleeding.

- Option 1: Cyanocobalamin: This is a synthetic form of vitamin B12. Its deficiency can cause anemia and neurological issues, but not scurvy.

- Option 2: Ascorbic acid: This is the correct answer. Vitamin C deficiency leads to scurvy.

- Option 3: Niacin: Also known as vitamin B3. Its deficiency causes pellagra, not scurvy.

- Option 4: Thiamine: This is vitamin B1. Its deficiency results in beriberi, not scurvy.