A characteristic shared by zinc and other transition elements is the ability to

Correct answer: form complex ions.

Explanation: Although zinc (Zn) is not a typical transition element (its Zn²⁺ is d¹⁰), it does share with transition metals the ability to form complex ions, e.g. [Zn(NH₃)₄]²⁺, [Zn(OH)₄]²⁻, and [ZnCl₄]²⁻.

• have variable oxidation states — Incorrect. Zinc is essentially fixed at +2 (no real variability).
• form complex ions — Correct. Zn²⁺ forms stable complexes with ligands.
• act as a catalyst — Not a characteristic feature for zinc; catalytic behaviour is typical for many transition metals but not notably for Zn.
• form coloured ions — Incorrect. Zn²⁺ is d¹⁰ and its aqueous ions are generally colourless.