Paramagnetic substance has unpaired electrons. More the number of unpaired electrons, more will be paramagnetic character.

Fe 2+ 1s 2 , 2s 2 , 2p 6 , 3s 2 , 3p 6 , 4s 0 , 3d 6

3d 6 means

∴ 4 unpaired electrons

Fe 3+ 1s 2 , 2s 2 , 2p 6 , 3s 2 , 3p 6 , 4s 0 , 3d 5

3d 5 means

∴ 5 unpaired electrons

Cr 3+ 1s 2 , 2s 2 , 2p 6 , 3s 2 , 3p 6 , 4s 0 , 3d 3

3d 3 means

∴ 3 unpaired electrons

Mn 3+ 1s 2 , 2s 2 , 2p 6 , 3s 2 , 3p 6 , 4s 0 , 3d 4 ,

3d 4 means

∴ 4 unpaired electrons

∴ Fe 3+ has highest number of unpaired electrons

∴ It is most paramagnetic.