Which of the following is an example of a polyatomic ion?

A polyatomic ion is a charged species composed of two or more atoms covalently bonded together. The correct answer, represented by the hydroxide ion (OH-), meets this definition as it consists of one oxygen atom and one hydrogen atom, which are bonded together and carry an overall negative charge.

In contrast, the other options are examples of monatomic ions. Sodium (Na+) and calcium (Ca2+) are both single atoms that have lost electrons, resulting in their positive charges. Chloride (Cl-) is also a monatomic ion formed when a chlorine atom gains an electron, giving it a negative charge. Thus, OH- stands out as the only choice that consists of multiple atoms bonded together, making it a polyatomic ion.