A species is said to  be diamagnetic when it has all the paired electrons.

Similarly if the species contain unpaired electron it is said to be paramagnetic.

To know the magnetic character of molecules we can use MO diagram.

When we draw MO diagram for dihydrogen anion ( H2-) we find one unpaired electron in antibonding sigma orbital. For dihydrogen cation (H2+) there is one unpaired electron in bonding sigma orbital. Dihydrogen molecule possess overall among them two electron located in sigma bonding M.O.and these are paired.

Di helium cation (He2+) possess three electron over all two are present in bonding sigma molecular orbital and are paired and one electron in antibonding sigma orbital which is unpaired.

From above discussion hope you have guessed that only dihydrogen H2 is diamagnetic species.

Rutuja,

For diamagnetic character, there should not be any unpaired electron in the molecules formation.

Here, for H2 the number of unpaired electron is 0. But in all other case there's an unpaired electron. Let it be H2- ,H2+ or He2+ ; there's an unpaired electron i.e, they are paramagnetic.

But in option 3 in H2 there's no unpaired electron. So, that is your answer as diamagnetic.

Hope this helps!