Final answer:
Parentheses in chemical symbols are used to represent ions, specifically polyatomic ions, within a chemical formula.
Step-by-step explanation:
The use of parentheses in chemical symbols is primarily to indicate the presence of polyatomic ions in a chemical formula. When multiple units of a polyatomic ion are present in a molecule, the ion is enclosed in parentheses, with a subscript to show the number of such ions. For example, the formula for calcium sulfate, which contains the sulfate ion (SO4) is written as CaSO4. However, if there are multiple sulfate ions, the formula would be written with parentheses, such as in calcium sulfate dihydrate, CaSO4·2H2O, meaning each molecule has two water molecules (H2O). Therefore, the correct answer to your question is that parentheses are used c) To represent ions in chemical formulas.