The correct option is Mg > Al > Zn > Fe.
Explanation
The reactivity of metals with dilute acids, such as hydrochloric acid, is determined by their position in the Reactivity Series (or Activity Series). Metals placed higher in the series are more electropositive and displace hydrogen from dilute acids more vigorously to form metal salts and hydrogen gas.
Analysis of the Metals:
- Magnesium (Mg): It is placed high in the reactivity series. It reacts quite vigorously with dilute hydrochloric acid, producing a rapid evolution of hydrogen gas bubbles.
- Aluminium (Al): It is placed below magnesium but above zinc. While it is inherently reactive, its reaction may initially be slow due to a protective oxide layer, but chemically it is more reactive than zinc and iron.
- Zinc (Zn): It is placed below aluminium. It reacts with dilute hydrochloric acid at a moderate rate.
- Iron (Fe): It is placed below zinc. It reacts slowly with dilute hydrochloric acid compared to the other three metals.
Therefore, the correct decreasing order of reactivity is Mg > Al > Zn > Fe.
Key Takeaway:
The standard order of reactivity for common laboratory metals is K > Na > Ca > Mg > Al > Zn > Fe > Pb > [H] > Cu. Metals above Hydrogen displace it from dilute acids, while those below do not.