The catalytic activity of transition metals and their compounds is ascribed mainly to:
their unfilled d - orbitals
their ability to adopt variable oxidation states
their chemical reactivity
their magnetic behavior
Many d-block elements and their complex are used as catalysts in different types of reactions due to variable oxidation states. Due to this transition metals can form and break complexes with the ligands or reactants and products to catalyze a reaction.
The correct answer is option 2.