The coloured nature of transition metal ions is associated with their partially filled

Correct answer: d-orbital

Explanation: Transition metal ions have partially filled d-orbitals. When ligands surround the metal ion, the d-orbitals split into levels of slightly different energies. Electrons can jump between these levels by absorbing visible light, and the remaining transmitted/reflected light gives the observed colour.

• f-orbital — Colours of lanthanoids/actinoids relate to f-orbitals, not typical transition-metal colours.
• s-orbital — Not responsible for the characteristic colours of transition ions.
• p-orbital — Not the main cause of transition-metal colours.
• d-orbital — Correct. d–d electronic transitions produce colour.