Reactivity series gives information on the reactive nature of element. Only more reactive element will replace a less reactive element in it compound. More reactive element usually displace less reactive element in it compound.
From the reaction above Y displace element Q from it compound. Element Q displace W from it compound. W displace Z in it compound. The element X cannot displace Z from it compound because it is less reactive .Their is no reaction.
Base on the reactivity element Y is the most reactive.
