The correct math is A-III, B-I, C-IV, D-II
A. has configuration, with 3 unpaired electrons. Hence, it shows paramagnetic behaviour. Complex of the type shows cis-trans isomerism.
B. has configuration, hence shows ' paramagnetic behaviour. Complex gives and ions in solution hence, shows ionisation isomerism.
C. has configuration but ligands are strong field ligands hence, it forms square planar complex. Thus, all electrons are paired and it also exhibits ionisation isomerism.
D. has configuration. But, ligands present are strong enough to cause electron pairing, hence, it shows diamagnetic behaviour and exhibits cis-trans isomerism as it is type complex.