The correct answer is option 3: C. Scurvy is a disease caused by a deficiency of vitamin C. Vitamin C, also known as ascorbic acid, is essential for the synthesis of collagen, a protein that helps in the formation of connective tissues, such as skin, blood vessels, and bones. When the body lacks vitamin C, collagen synthesis is impaired, leading to symptoms of scurvy.

Option 1: A (vitamin A) is not the correct answer. Vitamin A deficiency can cause other health problems, such as night blindness and dry skin, but it is not associated with scurvy.

Option 2: K (vitamin K) is not the correct answer. Vitamin K is essential for blood clotting, but deficiency in this vitamin does not cause scurvy.

Option 4: B12 (vitamin B12) is not the correct answer. Vitamin B12 deficiency leads to a condition called pernicious anemia, which is characterized by fatigue, weakness, and a decrease in red blood cells. It does not cause scurvy.

In conclusion, scurvy is caused by a deficiency of vitamin C (option 3).