The formula of aluminium oxide is $A{l}_2{O}_3$. If aluminium forms its chloride, what will be the formula of that compound?

The correct option is C ${\mathrm{AlCl}}_3$
In $A{l}_2{O}_3$, the valency of aluminium is 3 and oxygen is 2.

Valency of chlorine is 1.
So, for chloride of aluminium:

Symbol: Al Cl
Valency: 3 1

On interchanging the valencies of each element and writing them down as subscripts, we get the formula - ${\mathrm{AlCl}}_3$.