How do you balance $\mathrm{CaO}+{\mathrm{H}}_2\mathrm{O}\to \mathrm{Ca}{\left(\mathrm{OH}\right)}_2$ ?

The addition of a stoichiometric coefficient to the reactant and products is involved by balancing chemical equations.

In the given question the equation is the type of combination reaction and an exothermic reaction that results in giving off heat.

During balancing, count the element and make them equal on both sides.

In, $\mathrm{CaO}+{\mathrm{H}}_2\mathrm{O}\to \mathrm{Ca}{\left(\mathrm{OH}\right)}_2$ the reactant and product sides have an equal number of atoms.

We can see that

Here, we can see that the number of atoms on both sides is equal, creating a balanced equation.
Slaked lime is the reaction's end product, or (Ca(OH)₂). It serves a variety of functions, including adjusting water's pH to sterilise it and producing calcium carbonate through a reaction with carbon dioxide.

Therefore, The final balanced equation is $\mathrm{CaO}+{\mathrm{H}}_2\mathrm{O}\to \mathrm{Ca}{\left(\mathrm{OH}\right)}_2$. 

Hence, the correct option is (D).