Sample of 4 metals A, B, C and D were added to the following solutions separately. The results observed are shown below. What is the order of reactivities of these metals?

Metal	FeSO4	CuSO4	ZnSO4
A	No change	a green deposit	-
B	a grey deposit	a brown deposit	No change
C	-	No change	-
D	a brown deposit	-	A white coating on metal

Correct Option/Answer:
The correct option is C < A < B < D.

Understanding Displacement Reactions:
A displacement reaction occurs when a more reactive metal displaces a less reactive metal from its salt solution. If metal X reacts with a solution containing ions of metal Y and forms a deposit or coating, then metal X is more reactive than metal Y (X > Y). If no reaction occurs, metal X is less reactive than metal Y (X < Y).

Analyzing the Data for Each Metal:

1. Metal A:
- With ${\text{FeSO}}_4$: No change occurs. This implies metal A is less reactive than Iron (Fe):

$\text{A}<\text{Fe}$

- With ${\text{CuSO}}_4$: A green deposit is formed (displacement takes place). This implies metal A is more reactive than Copper (Cu):

$\text{A}>\text{Cu}$

Hence, metal A lies between Copper and Iron in terms of reactivity.

2. Metal B:
- With ${\text{FeSO}}_4$: A grey deposit is formed, showing metal B displaces Iron. Thus, metal B is more reactive than Iron (Fe):

$\text{B}>\text{Fe}$

- With ${\text{ZnSO}}_4$: No change occurs, showing metal B cannot displace Zinc. Thus, metal B is less reactive than Zinc (Zn):

$\text{B}<\text{Zn}$

3. Metal C:
- With ${\text{CuSO}}_4$: No change occurs. Since Copper sulphate solution is generally the easiest among these to undergo displacement by reactive metals, no reaction with ${\text{CuSO}}_4$ indicates that metal C is less reactive than Copper (Cu):

$\text{C}<\text{Cu}$

Therefore, metal C is the least reactive among all the metals tested.

4. Metal D:
- With ${\text{ZnSO}}_4$: A white coating on the metal is formed, indicating metal D displaces Zinc from its solution. Thus, metal D is more reactive than Zinc (Zn):

$\text{D}>\text{Zn}$

Since metal D can displace Zinc, which is the most reactive metal ion among the solutions (${\text{Zn}}^{2+}$), metal D is the most reactive metal overall.

Summary of Reactivity Order:
- Metal C is less reactive than Cu ($\text{C}<\text{Cu}$).
- Metal A is more reactive than Cu but less than Fe ($\text{Cu}<\text{A}<\text{Fe}$).
- Metal B is more reactive than Fe but less than Zn ($\text{Fe}<\text{B}<\text{Zn}$).
- Metal D is more reactive than Zn ($\text{Zn}<\text{D}$).

Combining these findings gives the overall increasing order of reactivity:

$\text{C}<\text{A}<\text{B}<\text{D}$