The correct answer is option 1: "Both the statements are individually true and statement II is the correct explanation of statement I."

Statement I states that Plaster of Paris hardens when mixed with water, which is true. Plaster of Paris is a building material that, when mixed with water, undergoes a chemical reaction known as hydration. This reaction causes the Plaster of Paris to harden and form a solid mass.

Statement II states that by combining with water, Plaster of Paris is converted into Gypsum, which is also true. Plaster of Paris is made from Gypsum by heating it to a high temperature and then grinding it into a fine powder. When water is added to Plaster of Paris, it reverts back to its original form, which is Gypsum.

Therefore, both statements are true, and statement II provides the correct explanation for why statement I is true. When Plaster of Paris is mixed with water, it undergoes a chemical reaction and is converted back into Gypsum, resulting in the hardening of the material.