Answer: Sc 3+.
- A Cu<sup>2+</sup>
- B Sc<sup>3+</sup>
- C Ni<sup>2+</sup>
- D Co<sup>2+</sup>
Correct answer: B. Sc<sup>3+</sup>
Explanation: Sc<sup>3+</sup> has a d<sup>0</sup> 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.