Q.The catalytic behaviour of transition metals and their compounds is ascribed mainly due to

Step 1. Recall the two structural reasons the chapter gives for transition-metal catalysis: (i) energetically available d orbitals that can bind a reactant molecule, and (ii) variable oxidation states that let the metal donate and accept electrons through a catalytic cycle.

Step 2. The specific worked example given -- V₂O₅ catalysing the oxidation of SO₂ to SO₃ in the manufacture of sulphuric acid -- is explicitly explained through vanadium's oxidation state changing from +5 (V₂O₅) to +4 (VO₂) and back, i.e. through the metal's variable oxidation states, not merely through orbital availability. …

Recall the chapter's stated structural explanation for catalytic activity and match it against the worked V₂O₅/Contact-Process exampl …

• Choosing 'unfilled d orbitals' (b) because it is also a genuine contributing factor, without noticing the question asks for the reason 'mainly' ascribed, which the textbook ties to variable oxidation states via its central worked example. …