Explanation:

Physical changes refer to changes in the physical properties of a substance without altering its chemical composition. In other words, the substance remains the same, but its physical appearance or state may change. Let's analyze each option to determine which one is not a physical change.

a) Rusting of iron:
When iron comes into contact with oxygen and moisture, it undergoes a chemical reaction called oxidation, forming a reddish-brown compound known as rust. This is an example of a chemical change because the iron is being chemically transformed into a new substance, rust.

b) Burning of wood:
When wood is burned, it undergoes a chemical reaction called combustion. The complex organic compounds present in wood are broken down into simpler substances, such as carbon dioxide, water vapor, and ash. This is also an example of a chemical change because the wood is being chemically transformed into different substances.

c) Melting of wax:
The melting of wax is indeed a physical change. Wax is a solid at room temperature, but when heat is applied, it melts and becomes a liquid. This change in state is purely physical because the chemical composition of the wax remains the same.

d) Cooking an egg:
Cooking an egg involves several chemical reactions. The heat denatures the proteins in the egg, causing them to unfold and coagulate, resulting in a solid cooked egg. This is a chemical change because the proteins in the egg are being chemically altered by the heat.

Conclusion:
Based on the explanations above, it is clear that the melting of wax (option c) is the only example of a physical change among the given options. The other options involve chemical reactions that result in the formation of new substances.