Step 1: Understanding the Question:
The question asks which vitamin, when lacking in the diet for a long time, leads to bleeding gums.
Step 2: Key Formula or Approach:
Bleeding gums point to weak connective tissue and fragile blood vessel walls in the mouth. We need the vitamin that directly builds and maintains this connective tissue.
Step 3: Detailed Explanation:
Vitamin C is needed to make collagen, the protein fibre that holds gum tissue firm and keeps the walls of small blood vessels strong. When Vitamin C is missing from the diet for weeks or months, collagen production drops, the gums become soft and swollen, and small blood vessels break easily. This condition is called scurvy, and bleeding gums along with loose teeth are its classic signs.
Now check the other options. A lack of Vitamin A mainly causes night blindness and dry, rough skin, not bleeding gums. A lack of Vitamin B vitamins usually causes problems like beriberi, pellagra, or anemia, again not gum bleeding as the main sign. A lack of Vitamin D mainly causes rickets in children and softening of bones in adults, since it disturbs calcium handling, not gum health.
Step 4: Final Answer:
Bleeding gums are the classic sign of Vitamin C deficiency, known as scurvy.
\[ \boxed{\text{Vitamin C}} \]