Answer : For a species to be diamagnetic it needs to have no unpaired electrons and for paramagnetic the species needs t have unpaired electrons.

Answer 1) :  C_{2} ^{+2}

Electronic configuration of carbon is  1s^{2}2s^{2} 2p^{2}

So, electronic configuration of  C_{2} ^{+2} will be  1s^{2} 2s^{2} , as it has given its two electrons present in the outermost valence shell.

This molecule of  C_{2} ^{+2} will behave as diamagnetic species as there are no unpaired electrons in the orbital.

Answer 2)  Li^{-2}

Explanation : The electronic configuration of Lithium is  1s^{2} 2s^{1}

When lithium is in  Li^{-2} state it will have electronic configuration as  1s^{2} 2s^{2} 2p^{1}

This shows that Lithium has one electron which is available in 2p orbital, and it is also unpaired.

Therefore, the  Li^{-2} species will behave like a paramagnetic substance.

Answer 3)  B_{2}^{-2}

Boron has electronic configuration as  1s^{2} 2s^{2} 2p^{1} ,

So, the excited state of  B_{2}^{-2} will be  1s^{2} 2s^{2} 2p^{3} ,

Here, Boron in excited state has one unpaired electron in 2p orbital rest 2 electrons are paired in the 2p orbital.

This will make  B_{2}^{-2} as a paramagnetic substance.