K>Ca>Mg>Al .
The reactivity of elements depends upon their tendency to lose or gain electrons to complete their outer most shell. On moving left to right in a period reactivity decreases and down the group increases. So Ca and Mg both belongs to 2‌nd ‌ group but have different periods. Mg is in 3‌rd ‌ period and Ca is in 4‌th ‌ period.