Answer: Mercury

Sodium  can be easily cut with a knife

Silver is the best conductor of heat.

Mercury and Lead are poor conductor of heat

Malleable:Substances that can be beaten into thin sheets are called malleable. For example, most of the metals are malleable.
Ductile: Substances that can be drawn into thin wires are called ductile. For example, most of the metals are ductile.

Sodium is high reactive element. If it is kept in open it can explosively react with oxygen to catch fire. Hence to prevent accidental damage sodium is immersed in kerosene oil.

(i) 3Fe (s) + 4H₂O (g) → Fe₃O₄ (aq) + 4H₂ (g)

(ii) Ca (s) + 2H₂O (l) → Ca(OH)₂ (aq) + H₂ (g) + Heat
     2K (s) + 2H₂O (l) → 2KOH (aq) + H₂ (g) + Heat

Solution:

A + FeSO₄ → No reaction, i.e., A is less reactive than iron

A + CuSO₄ → Displacement, i.e., A is more reactive than copper

B + FeSO₄ → Displacement, i.e., B is more reactive than iron

B + ZnSO₄ → No reaction, i.e., B is less reactive than zinc

C + FeSO₄ → No reaction, i.e., C is less reactive than iron

C + CuSO₄ → No reaction, i.e., C is less reactive than copper

C + ZnSO₄ → No reaction, i.e., C is less reactive than zinc

C + AgNO₃ → Displacement, i.e., C is more reactive than silver

D + FeSO₄/CuSO₄/ZnSO₄/AgNO₃ → No reaction, i.e., D is less reactive than iron, copper, zinc, and silver

(i) B is most reactive metal.
(ii) B will displace copper from copper sulphate.
(iii) Arrangement of metals in the order of decreasing reactivity B>A>C>D.