Among the given species only K₂CrO₄ is diamagnetic as central metal atom Cr in it has [Ar]3d⁰ electronic configuration i.e., all paired electrons. The structure and oxidation state of central metal atom of this compound are as follows

Structure

Oxidation state Cr⁶⁺

Rest all the compounds are paramagnetic. Reasons for their paramagnetism are given below.

(i) H-atom have 1s¹ electronic configuration i.e., 1 unpaired electron.

(ii) NO₂ i.e.

in itself is an odd electron species.

(iii) O$$_2^ - $$ (Superoxide) has one unpaired electron in $$\pi$$* molecular orbital.

(iv) S₂ in vapour phase has O₂ like electronic configuration i.e., have 2 unpaired electrons in $$\pi$$* molecular orbitals.

(v) Mn₃O₄ has following structure

Thus, Mn is showing +2 and +4 oxidation states. The outermost electronic configuration of elemental Mn is 3d⁵4s². Hence, in both the above oxidation states it has unpaired electron as

(vii) (NH₄)₂NiCl₄ has Ni as central metal atom with +2 oxidation state. The electronic configuration of Ni²⁺ in the complex is

(viii) In K₂MnO₄ central metal atom Mn has +6 oxidation state with following structure

Electronic configuration of Mn⁶⁺ is