Heating an aqueous solution of aluminium chloride to dryness will give

Aluminium chloride is a largely covalent chloride because the highly charged, small $$Al^{3+}$$ ion polarises the chloride ions. When such covalent chlorides are dissolved in water, they undergo extensive hydrolysis.

Step 1 - Hydrolysis in water:
$$AlCl_3 + 3\,H_2O \;\longrightarrow\; Al(OH)_3 + 3\,HCl$$

During this step, hydrochloric acid escapes as a gas (or stays in solution and can later be boiled off), leaving gelatinous aluminium hydroxide behind.

Step 2 - Heating to dryness:
On strong heating, the hydroxide loses water (dehydration):
$$2\,Al(OH)_3 \;\xrightarrow{\;\text{heat}\;} Al_2O_3 + 3\,H_2O$$

The final solid residue after all water has been driven off is aluminium oxide, $$Al_2O_3$$.

Hence, heating an aqueous solution of aluminium chloride to dryness ultimately gives aluminium oxide.

Option C which is: $$Al_2O_3$$