Option 1: The melting of an ice cube is an example of a physical change, not a chemical change. When ice melts, it undergoes a phase change from solid to liquid, but the chemical composition of water remains the same.

Option 2: The boiling of gasoline is also a physical change. Boiling is the process in which a liquid changes into a gas due to the absorption of heat. In this case, gasoline undergoes a change in state from liquid to gas, but its chemical composition remains unchanged.

Option 3: The frying of an egg is an example of a chemical change. When an egg is fried, the heat causes the proteins in the egg to denature, leading to the formation of new compounds. The chemical composition of the egg changes as the proteins react to the heat, resulting in a cooked egg with different properties than a raw egg.

Option 4: The attraction of an iron nail to a magnet is a physical property and not an example of a chemical change. It is a result of the magnetic field exerted by the magnet on the iron nail.

Therefore, the correct answer is option 3, as it describes a process that involves a chemical change, unlike the other options.