Magnetic behaviour of complex compounds
Magnetic behaviour of complex compounds are determined on the basis of magnetic moment, rather the extent of paramagnetism is measured in terms of the magnetic moment μ.  

μ = √n(n+2) BM , where n notates the number of unpaired electrons. 

First lets give a look towards the number of unpaired electron each of the central metal atom has:




Here the question is asking to determine the complex compound which shows the lowest paramagnetic behaviour. As Co^3+ has no unpaired electron, its μ = 0, hence it is diamagnetic, and hence shows shows lowest paramagnetic behaviour among the above complexes.
And best of luck for jee'17.
Me too appearing the exam.

Hope it helps