Rishi dropped metals $P,Q,R$and $S$ in the salt solution and recorded the following observations.

Based on the above observations :

A. Arrange the given samples in the decreasing order of reactivity.

B. If $Q$ reacts with $CuS{O}_4$, solution what will be the reaction taking place?

A. 

S will have the minimum reactivity because no reaction occurs, and Q will have the maximum reactivity as it reacts with all four metals. Q removes all metals from their salt solution; as a result, it has the highest level of reactivity, followed by P, which displaces Ca and Zn from its salt solution, then R, which only removes Zinc from its salt solution, followed by S has the least reactivity of all the compounds. 

Hence it didn't replace any metal in the salt solution. Thus, the decreasing order of reactivity is as follows:

$𝑄 > 𝑃 > 𝑅 > 𝑆$

B.

The chemical formulae of products formed when Q reacts with $CuS{O}_4$ are Cu and $QS{O}_4$. Since $Q$ has the highest level of reactivity and can remove even $Na$ from its salt solution, it will definitely replace $Cu$ because it is less reactive than $Na$

$CuS{O}_4+Q\to Cu+QS{O}_4$

Since the valency of the product is not provided, the chemical formulae of products formed can also be $Q_{2}S{O}_4$and so on.