How many of the following species show paramagnetism?
${\mathrm{Na}}^{+},{\mathrm{O}}_2,{\mathrm{Cu}}^{2+},{\mathrm{Fe}}^{2+},{\mathrm{Zn}}^{2+},{\mathrm{Cr}}^{3+},{\mathrm{Cu}}^{+},{\mathrm{O}}_2^{-},{\mathrm{O}}_2^{2-}$

Species with at least one or more unpaired electron are attracted by the magnetic field and are said to be paramagnetic.
${\mathrm{Na}}^{+},{\mathrm{Zn}}^{2+},{\mathrm{Cu}}^{+}\text{and }{\mathrm{O}}_2^{2-}$ 

No unpaired electron hence, diamagnetic.
O₂-  two unpaired electrons in anti bonding molecular orbitals.
Fe²⁺-Four unpaired electrons in 3d-orbitals 

Cr³⁺ -Three unpaired electrons in 3d-orbital 

Cu⁺² -One unpaired electron in 3d-orbital.
${\mathrm{O}}_2^{-}$-one unpaired electron in antibonding molecular orbitals.