Iodine solution is commonly used as a test for the presence of starch. When iodine comes into contact with starch, it forms a bluish-black color. This reaction occurs due to the formation of a complex between iodine molecules and the starch molecules. Starch is a complex carbohydrate composed of long chains of glucose molecules, and it is commonly found in foods such as potatoes, rice, and bread.

The other options mentioned are not accurate. Iodine solution does not react with sugar, proteins, or fats in the same way it reacts with starch. Therefore, option 1 (sugar), option 2 (proteins), and option 4 (fats) are incorrect. It is important to note that different chemical tests are used to detect the presence of these other nutrients.