Which of the following is a polyatomic ion?

The correct option is B

${\mathrm{PO}}_4^{3-}$

Explanation of the correct option:

The correct option is B:

1. The polyatomic ions are formed when 2 or more atoms lose or gain electrons in a molecule.
2. ${\mathrm{PO}}_4^{3-}$ ion has 1 Phosphorous $\left(\mathrm{P}\right)$ atom and 4 Oxygen $\left(\mathrm{O}\right)$ atoms.
3. Thus, ${\mathrm{PO}}_4^{3-}$ is a polyatomic ion.

Explanation of the incorrect options:

Options A, C, and D:

1. The ions ${\mathrm{Zn}}^{2+}$, ${\mathrm{Mg}}^{2+}$, and ${\mathrm{Cr}}^{3+}$ have only one type of atom, that is Zinc $\left(\mathrm{Zn}\right)$, Magnesium $\left(\mathrm{Mg}\right)$, and Chromium $\left(\mathrm{Cr}\right)$ respectively.
2. Thus, these ions are called monoatomic ions.

Hence, option (B) is correct.
${\mathrm{PO}}_4^{3-}$ is a polyatomic ion.