Which of the following is a combination reaction?

${\text{(i)\hspace{0.17em}2KClO}}_3\stackrel{\mathrm{heat}}{\to }\text{\hspace{0.17em} 2KCl\hspace{0.17em}}+{\text{\hspace{0.17em}3O}}_2$

$\left(\mathrm{ii}\right)\mathrm{Mg}{\left(\mathrm{OH}\right)}_2 \to \mathrm{MgO}+{\mathrm{H}}_2\mathrm{O}$

$\left(\mathrm{iii}\right)4\mathrm{Al}+3{\mathrm{O}}_2\to 2{\mathrm{Al}}_2{\mathrm{O}}_3$

$\left(\mathrm{iv}\right)\mathrm{Zn}+{\mathrm{FeSO}}_4\to {\mathrm{ZnSO}}_4+\mathrm{Fe}$

A combination reaction is a reaction in which two or more substances combine to form a single new substance. 

$4\mathrm{Al}+3{\mathrm{O}}_2\to 2{\mathrm{Al}}_2{\mathrm{O}}_3$

It is only in the third reaction that two or more reactants (aluminium and oxygen) combine to form a single product (Aluminium oxide). So it is an example of a combination reaction.