When calcium oxide is dissolved in water calcium hydroxide is formed. Which law of the following law is obeyed here?

The correct option is B Law of conservation of mass
According to the law of chemical combination, mass can neither be created nor destroyed in a chemical reaction. In the above reaction calcium oxide and water formed calcium hydroxide, so the mass of reactants and products remain the same.

Law of chemical combination is also known as law of conservation of mass.

Hence, it obeys the rule of law of conservation of mass.

Hence, the correct option is $A$