Why are brackets used in the formula Ca(OH)₂?

The correct option is "To indicate two hydroxide groups".

In chemical formulas, parentheses (or brackets) are used to group together polyatomic ions that act as a single unit. In this compound, calcium hydroxide, the hydroxide group ($\text{OH}$) is a polyatomic ion consisting of one oxygen atom and one hydrogen atom bound together, carrying a net charge of $-1$.

Since the calcium ion (${\text{Ca}}^{2+}$) has a charge of $+2$, two hydroxide ions are required to balance the charge and make the compound electrically neutral.
The subscript $2$ placed outside the parentheses in ${\text{Ca(OH)}}_2$ indicates that the entire hydroxide group inside the brackets is multiplied by two, meaning there are two distinct hydroxide groups bonded to each calcium atom.