The species that can have a trans-isomer is:
( en = ethane-1,2-diamine, ox = oxalate)
: hybridised , therefore no trans-isomer is possible.
: Bidentate ligand will be co-ordinating at in square planar complex.
Thus, no trans-isomer is possible.
: In this compound as well, no trans-isommer is possible.
In this compound both trans and cis are possible.
option (4) is correct.