What is a polyatomic ion?

Question

$\begin{array}{1 1} \text{a.Ions made of 3 types of atom that act as 2 ions} \\ \text{ b. Ions that have 2 or more atoms} \\ \text{ c. Ions that are made of more than 1 type of atom and act as one} \\ \text{ d. Ions that have 5 different types of atoms} \end{array}$