Answer: Sc3+.
- A Cu2+
- B Sc3+
- C Ni2+
- D Co2+
Correct answer: B. Sc3+
Explanation: Sc3+ has a d0 configuration, so no d-d transition is possible and it is colourless. The others have partly filled d orbitals and are coloured.
Aqueous solutions of transition metal ions show characteristic colours caused by electrons absorbing visible light to jump between split d-orbitals; the exact colour depends on the metal, its oxidation state, and surrounding ligands.
Concept context
Transition metals (d-block) and lanthanides/actinides (f-block). Known for coloured compounds, variable oxidation states, complex formation, and catalytic properties. Frequently tested in JEE and NEET.