Physical Change:
A physical change refers to a change in the physical properties of a substance without altering its chemical composition. In other words, the substance remains the same at a molecular level even though its appearance or state may change. Examples of physical changes include changes in state (solid to liquid to gas), changes in shape or size, and changes in appearance.

Options:
a) Burning of paper: When paper burns, it undergoes a chemical change known as combustion. The chemical composition of paper changes as it reacts with oxygen to produce carbon dioxide, water vapor, and other byproducts. Therefore, burning of paper is not a physical change.

b) Rusting of iron: Rusting is a chemical change that occurs when iron reacts with oxygen and moisture in the presence of air. The chemical composition of iron changes as it forms iron oxide (rust). Hence, rusting of iron is not a physical change.

c) Melting of ice: When ice melts, it changes from a solid state to a liquid state. The water molecules in ice rearrange themselves and gain enough energy to overcome the intermolecular forces holding them in a fixed pattern. However, the chemical composition of water remains the same during this change. Therefore, melting of ice is a physical change.

d) Digestion of food: Digestion is a complex process that involves the breakdown of food into smaller molecules through chemical reactions. Enzymes in the digestive system break down carbohydrates, proteins, and fats into simpler compounds that can be absorbed by the body. This process results in the chemical transformation of food substances and is therefore a chemical change.

Conclusion:
Based on the given options, the melting of ice is the only example of a physical change. Burning of paper, rusting of iron, and digestion of food all involve chemical changes where the chemical composition of the substances involved is altered.