Answer:
 Magnesium
 Zinc 
 Iron 
 Copper
Step-by-step explanation:
The given elements are: 
 Magnesium, copper, iron and zinc 
 We are to put them in the order from the least reactive to the most reactive of all. 
To solve this problem, we simply use the activity series of metals to arrange the elements. 
The activity series arranges metals from the most reactive to the least reactive. The more reactive metals displaces the lesser one from solutions. 
 So; 
  Magnesium
 Zinc 
 Iron 
 Copper