Question

what are polyatomic ions?
what is the formula for the chlorate
ion?
cl-
clo-
clo3-

Explanation:

Step1: Analyze the options

• $\ce{Cl-}$ is the chloride ion, not chlorate.
• $\ce{ClO-}$ is the hypochlorite ion.
• $\ce{ClO3-}$ follows the naming convention for chlorate. In polyatomic ions, the -ate suffix generally corresponds to a certain number of oxygen atoms. Chlorate has three oxygen atoms.

Answer:

$\ce{ClO3-}$ (the third option)