Explanation:

- Option 1: Tooth decay

- Tooth decay is typically caused by poor oral hygiene, sugary diets, and lack of fluoride, rather than iodine deficiency.

- Option 2: Swelling in the neck

- This swelling, known as goiter, is indeed a visible sign of severe iodine deficiency.

- Iodine is crucial for thyroid function, and deficiency can cause the thyroid gland to enlarge.

- Correct Answer: Swelling in the neck.

- Option 3: Loss of vision in darkness

- This condition is called night blindness and is usually due to vitamin A deficiency, not iodine.

- Option 4: Bleeding gums

- Bleeding gums are often associated with a deficiency of vitamin C, leading to scurvy.