Aluminium chloride in acidified aqueous solution forms an ion having geometry:

🧠 Al³⁺ in Water = Hexaaqua Cation

When $\mathrm{AlCl_3}$ dissolves in acidified water, $\mathrm{Al^{3+}}$ doesn't stay bare — it pulls in six water molecules to form $[\mathrm{Al(H_2O)_6}]^{3+}$.

Six donor atoms → octahedral. The three ionised Cl⁻ become spectators in solution.

🗺️ Why CN = 6 for Al³⁺

$\mathrm{Al^{3+}}$ has a small radius (~54 pm) and a high charge (+3) — strongly attractive to water lone pairs. Six waters pack around it without crowding → CN = 6, octahedral. This is the aqua form of aluminium.

⚡ The "Hard, Small, +3 → Octahedral" Pattern

Hard cations like $\mathrm{Al^{3+}}, \mathrm{Fe^{3+}}, \mathrm{Cr^{3+}}, \mathrm{Ga^{3+}}$ in water all give octahedral hexaaqua ions. Lock this.

⚠️ Don't Pick Tetrahedral

A common slip: students associate $\mathrm{AlCl_4^-}$ (tetrahedral, in non-aqueous melt) with the aqueous form. They are different species. In water, Al goes hexaaqua octahedral; in molten conditions or vapor phase, $\mathrm{Al_2Cl_6}$ dimer or $\mathrm{AlCl_4^-}$ tetrahedral exists.

$\boxed{\text{Answer: (1) Octahedral}}$