The d & f Block Elements — Chemistry STD 12 Science — Question

4. All of the above.

Explanation:
Transition metals  has incompletely filled d orbital. They have vacant orbitals to accept coordination bond. Complex compounbs are those in which the metal ion bind a number of anions or neutral molecules giving complex species with characteristics properties. The transition metals form a large no. of complex compounds.
This is due to the comparatively smaller size of the metal ions,their high ionic charges and the availability of d orbitals for bond formation.