Which of the following complexes are heteroleptic?
Heteroleptic complexes are attached to more than one kind of ligand. Here, in Option B[Fe(NH3)4 Cl2]+ and Option D[Co(NH3)4Cl2] are attached to two different ligands each and hence they are heteroleptic complexes.
Whereas, in option A and C, there is only one type of ligand is connected with Central Atom.
Thus, Options B & D are the correct answers.