Step 1: Recall what a catalyst does.
A catalyst speeds up a reaction by giving an easier path with lower activation energy, without being used up itself.
Step 2: First special feature of transition metals.
They show variable oxidation states. This lets them accept and give back electrons easily, helping them form intermediate compounds with the reactants.
Step 3: How that helps.
By forming these intermediates, the transition metal opens a new low-energy route for the reaction, so it goes faster.
Step 4: Second special feature.
Their partly filled d-orbitals and free surface valencies can adsorb reactant molecules onto the metal surface, bringing them close together and weakening their bonds.
Step 5: Combine the reasons.
Both the change in oxidation state and the ability to adsorb reactants make transition metals good catalysts.
Answer: Transition metals are good catalysts because they have variable oxidation states (so they form intermediate compounds and provide a lower-energy path) and partly filled d-orbitals that adsorb reactant molecules on their surface, both of which lower the activation energy.