The correct answer is option 4. The color of cow`s milk is slightly yellow due to the presence of carotin. Carotin is a class of pigments that are commonly found in plants and fruits, such as carrots and tomatoes. When cows consume grass or other green feed, their bodies convert the carotin into Vitamin A, which is then secreted into the milk. This gives the milk a slightly yellow color. Xanthophyll, option 1, is another class of pigments found in plants, but it is not responsible for the color of cow`s milk. Riboflavin, option 2, and ribulose, option 3, are both compounds that are not related to the color of cow`s milk. Therefore, option 4 carotin is the correct answer.