The property of metal that makes it suitable as a catalyst is

1. Analyze the question The question asks which electronic feature makes a metal (typically a transition metal such as Fe, Ni, Pt, V, or Mn) a good catalyst.

2. Explain the concept Transition metals have partially filled d-orbitals. This gives them two useful properties:

• Variable oxidation states. They can easily gain or lose electrons, so they can form unstable intermediate compounds with reactants and then regenerate themselves.
• Vacant or partly filled d-orbitals. Reactant molecules can bond to the metal surface (adsorption), which weakens the bonds in the reactants and lowers the activation energy.

3. Step-by-step solution

• Examples of catalysts: Fe in the Haber process, V₂O₅ in the Contact process, Ni in hydrogenation of oils, MnO₂ in decomposition of H₂O₂.
• All of these are d-block (transition) elements, and all have partially filled d-orbitals.
• Option D (filled d-orbital) describes Zn, Cd and similar elements. These are not true transition metals and are poor catalysts because they lack variable oxidation states.
• Options A and C (f-orbitals) concern the lanthanides and actinides, which are not the usual catalysts at this level.

4. Answer The correct option is B. partially filled d-orbitals.

5. Exam tip Link the words "catalyst" and "transition metal" to "partially filled d-orbitals" and "variable oxidation states." If you see these in the options, pick them.