C) cooking of food

this is because when food is cooked it can't go back to uncooked food

Chemical Change: Cooking of Food

Explanation:
Chemical change refers to a change that occurs when the identity of the substance changes, resulting in the formation of a new substance with different chemical properties. Chemical changes are irreversible, and they involve breaking and forming of chemical bonds.

The options given in the question are physical changes except for cooking of food. Cooking of food involves various chemical changes such as denaturation of proteins, caramelisation of sugars, hydrolysis of starch, etc. Let's understand these changes in detail.

Denaturation of Proteins:
When food is heated, the proteins present in it undergo denaturation. It is a process in which the protein molecule loses its three-dimensional structure and its biological activity. The denaturation of proteins is a chemical change as it involves breaking of chemical bonds, resulting in the formation of new compounds.

Caramelisation of Sugars:
When sugar is heated, it undergoes a chemical change called caramelisation. In this process, the sugar molecules break down into smaller molecules, resulting in the formation of a brown colour and a characteristic aroma. Caramelisation is a chemical change as it involves the breaking and forming of chemical bonds.

Hydrolysis of Starch:
Starch is a complex carbohydrate that gets hydrolysed in the presence of heat and moisture. Hydrolysis is a chemical change as it involves the breaking of chemical bonds. Starch gets converted into simple sugars such as glucose and fructose, which are more easily digestible.

Conclusion:
All the options given in the question except cooking of food are physical changes. Cooking of food involves various chemical changes such as denaturation of proteins, caramelisation of sugars, and hydrolysis of starch. Hence, cooking of food is a chemical change.