Which of the following metallic oxides is most reactive towards HCl?

As we move down in group 1, the reactivity of metallic oxides increases and across a period, the reactivity of metallic oxides decreases. Na and K belong to group 1, Mg belongs to group 2 and Zn belongs to group 12. So, Group 1 elements are more reactive than Mg and Zn. Between Na and K, as potassium is present below sodium in group-1 ,So K₂O will be the most reactive metallic oxide.

Metallic oxides react with acids to form salt and water,

${\mathrm{K}}_2\mathrm{O}+2\mathrm{HCl}\to 2\mathrm{KCl}+{\mathrm{H}}_2\mathrm{O}$