The correct answer is Y > X > Z.Concept:Displacement reaction: It is a chemical reaction wherein a more reactive element displaces the less reactive one from its salt solution.For example, CuSO4 + Fe ? FeSO4 + Cu, here Fe is more reactive than Cu.Explanation:The reaction of element Y with XSO4 :Y + XSO4 ? YSO4Y being more reactive than X displaces it from its aqueous solution.The reaction of element X with ZSO4 :X + ZSO4 ? ZSO4X being more reactive than Z displaces Z from its aqueous solution.The correct order of reactivity of 'X', 'Y' and 'Z' is Y > X > Z.