The correct answer to the question is option-2: Iodine deficiency. Goitre, which is the enlargement of the thyroid gland, is primarily caused by a deficiency of iodine in the diet. The thyroid gland requires iodine to produce thyroid hormones, which are essential for the regulation of metabolism and growth in the body.

Option-1: Sodium deficiency does not directly cause the development of goitre. While sodium is essential for fluid balance and nerve function, it does not play a direct role in thyroid hormone production or the enlargement of the thyroid gland.

Option-3: Calcium deficiency is not a primary cause of goitre. Calcium is important for bone health and muscular function, but it does not have a direct link to thyroid gland enlargement.

Option-4: Iron deficiency is also not directly related to the development of goitre. Iron is essential for red blood cell production and oxygen transport in the body, but it does not play a role in thyroid hormone synthesis or goitre formation.

In conclusion, goitre is mainly caused by a deficiency of iodine in the diet.