The correct answer is option 4, Melanin. Melanin is a pigment that is responsible for the color of human skin. It is produced by cells called melanocytes, which are found in the epidermis, the outermost layer of the skin. Melanin absorbs the harmful ultraviolet (UV) radiation from the sun and helps protect the skin from its damaging effects. The amount of melanin produced by melanocytes determines the color of an individual`s skin. People with more melanin have darker skin, while those with less melanin have lighter skin. Haemoglobin, which is mentioned in option 1, is a protein in red blood cells that carries oxygen, but it is not directly involved in the color of the skin. Insulin and adrenaline, mentioned in options 2 and 3, are hormones that play roles in maintaining blood sugar levels and regulating the body`s response to stress, respectively, but they do not affect skin color.