The correct answer is option 4: unsaturated fatty acids. Drying oils are vegetable oils that have the ability to dry and harden when exposed to air. This property is due to the high content of unsaturated fatty acids in these oils, specifically polyunsaturated fatty acids. Unsaturated fatty acids contain one or more double bonds in their chemical structure, which makes them more reactive than saturated fatty acids. When these oils are exposed to air, the double bonds undergo a process called oxidation, which leads to the formation of a film or solid coating. This makes drying oils an ideal ingredient in products such as paints, varnishes, and wood finishes. It is important to note that drying oils should not be confused with fats, proteins, or saturated fatty acids, as they have distinct chemical compositions and properties.