How many of the following species are paramagnetic? sc3+ br- mg2+ se?

           One: Selenium is Paramagnetic

Explanation:
                   Those compounds which have unpaired electrons are attracted towards magnet. This property is called as paramagnetism. Lets see why remaining are not paramagnetic.

Electronic configuration of Scandium;

Sc  =  21  = 1s², 2s², 2p⁶, 3s², 3p⁶, 4s², 3d¹

Sc³⁺  =  1s², 2s², 2p⁶, 3s², 3p⁶ 

Hence in Sc³⁺ there is no unpaired electron.

Electronic configuration of Bromine;

Br  =  35  = 1s², 2s², 2p⁶, 3s², 3p⁶, 4s², 3d¹⁰, 4p⁵

Br⁻  =  1s², 2s², 2p⁶, 3s², 3p⁶, 4s², 3d¹⁰, 4p⁶

Hence in Br⁻ there is no unpaired electron.

Electronic configuration of Magnesium;

Mg  =  12  = 1s², 2s², 2p⁶, 3s²

Mg²⁺  =  1s², 2s², 2p⁶

Hence in Mg²⁺ there is no unpaired electron.

Electronic configuration of selenium;

Se  =  34  = 1s², 2s², 2p⁶, 3s², 3p⁶, 4s², 3d¹⁰, 4p⁴

Or,

Se  =  34  = 1s², 2s², 2p⁶, 3s², 3p⁶, 4s², 3d¹⁰, 4px², 4py¹, 4pz¹

Hence in Se there are two unpaired electrons hence it is paramagnetic in nature.