Diamagnetism is exhibited by
 $ A) $ Cobalt
 $ B) $ Water
 $ C) $ Oxygen
 $ D) $ Iron $ $

Hint :Diamagnetic susceptibility of a compound is the sum of the susceptibility of its components. However, the parts of a diamagnetic susceptibility is concerned is probably its electron bonds rather than its atoms and their electrons. Accordingly, the contribution of an atom to a compound depends upon other constituents of the compound.

Complete Step By Step Answer:
Because water has a relative magnetic permeability less than or equal to $ 1 $ , and along these lines magnetic susceptibility is not exactly or equivalent to $ 0 $ . This implies that diamagnetic materials are repulsed by magnetic fields.
Option $ A) $ is incorrect.
 Because the unpredictable particle $ {[Co{({H_2}O)_6}]^{3 + }} $ has $ Co $ in the $ + 3 $ oxidation state, which means it has an electron arrangement of $ [Ar]4{s_0}3{d_6} $ . Consequently it has $ 4 $ unpaired electrons and would be paramagnetic. $ $
Option $ C) $ is incorrect.
Because as indicated by molecular orbital theory, there is a $ 1 $ unpaired electron in the π $ 2py $ antibonding orbital. As particles containing unpaired electrons are strongly pulled in by magnetic fields, subsequently oxygen has paramagnetic nature.
Option $ D) $ is incorrect.
Because the lasting second for the most part is because spin if unpaired electrons in molecular atomic orbital. In the event that there is adequate energy trade between adjoining dipoles they will cooperate, and maybe immediately adjust and form magnetic domains, resulting in ferromagnetism.
Correct option is $ (B) $ .

Note :
Diamagnetism is a weak type of attraction that is prompted by the adjustment in the orbital movement of electrons because of an applied magnetic field. This magnetism is not permanent and perseveres just within the sight of an outside field.