The correct answer is option 3 - hydration leading to another hydrate.

When Plaster of Paris (calcium sulfate hemihydrate) is mixed with water, it undergoes a chemical reaction called hydration. This reaction involves the binding of water molecules with calcium sulfate hemihydrate, resulting in the formation of a new compound called calcium sulfate dihydrate or gypsum.

During hydration, each molecule of calcium sulfate hemihydrate combines with two molecules of water to form a crystal lattice structure of calcium sulfate dihydrate. This process causes the material to harden and set, transforming it from a powder into a solid mass.

Option 1, dehydration, is incorrect because the setting of Plaster of Paris involves the addition of water, not the removal of water.

Option 2, oxidation with atmospheric oxygen, is incorrect because the setting reaction is not caused by the reaction of Plaster of Paris with oxygen in the air.

Option 4, combination with atmospheric CO2 (carbon dioxide), is incorrect because the setting of Plaster of Paris does not involve the reaction with carbon dioxide.

So, the correct explanation is that the setting of Plaster of Paris is achieved through a process of hydration, where water molecules bind with calcium sulfate hemihydrate to