Aluminium ion properties and reactivity

Steps:

• Aluminium is in Group 13 (p-block), not a transition metal, so its Al³⁺ ion has no d-electrons and is colourless in aqueous solution.
• Transition metal ions are coloured due to d-d transitions, but main group ions like Al³⁺ are not.
• Electrolysis extracts aluminium because it is too reactive to be reduced by carbon, placing it high in the reactivity series (above magnesium).
• Thus, gaps are filled by "colourless" for [1] and "high" for [2].

Why C is correct:

• Al³⁺ ions are colourless as they lack unpaired d-electrons for light absorption (definition of ion color in coordination chemistry); electrolysis confirms high reactivity per the reactivity series law.

Why the others are wrong:

• A: Aluminium ions are colourless, not coloured.
• B: Aluminium is high, not low, in reactivity.
• D: Aluminium is high, not low, in reactivity.

Final answer: C