Melting of butter is a physical change.

Explanation:
A physical change is a change in the physical properties of a substance, such as its shape, size, or state, without changing its chemical composition. In the case of melting butter, the butter changes from a solid state to a liquid state without undergoing any chemical reactions.

Here is a detailed explanation of why the other options are not physical changes:

a) Changing of wheat to bread: This is a chemical change because the wheat undergoes a process called baking, where it combines with other ingredients and undergoes a chemical reaction to form bread. The composition of the wheat changes, so it is not a physical change.

b) Rusting of iron: Rusting is a chemical change. When iron reacts with oxygen and moisture in the air, it forms iron oxide (rust). This is a chemical reaction that changes the composition of the iron, so it is not a physical change.

c) Melting of butter: This is a physical change. Butter is a solid at room temperature, but when heated, it melts and becomes a liquid. The molecules in the butter rearrange themselves, but the chemical composition of the butter remains the same.

d) Burning of paper: Burning is a chemical change. When paper burns, it undergoes a combustion reaction where it reacts with oxygen in the air and releases heat, light, and gases. The original paper is converted into ash, gases, and smoke, so it is not a physical change.

In summary, the melting of butter is a physical change because it only involves a change in the physical state of the substance without altering its chemical composition. The other options involve chemical changes where the composition of the substances changes.