Which of the following is the correct arrangement of the given metals in ascending order of their reactivity?

Zinc, Iron, Magnesium, Sodium

As we move across the period the standard reduction of metals increases which means metals that are present on the right side of the periodic table can gain electrons easily and can form neutral ions. But between zinc and iron, iron has a higher reduction potential. Hence it accepts electrons easily as compared to zinc. So the correct order of reactivity is: 

Sodium > Magnesium > Zinc > Iron