Shouldn't the answer be (1)
Q.9. The correct statement
(1) Green vitriol and blue vitriol are isomorphus
(2) $KMn{O}_4 and K_{2}C{r}_2{O}_7$ are coloured due to d-d transitions
(3) $C{u}_{2}C{l}_2 and A{g}_{2}S$ are coloured
(4) Upon strong heating paramagnetic gases are evolved by $NaN{O}_3 and AgN{O}_3$
Sol. Answer (4)

Dear Student,
The answer is option 4. When Sodium nitrate is heated it liberated oxygen which is a paramagnetic gas.When silver nitrate is heated it libertes nitrogen oxide which is also a paramagnetic gas.

Now considering the option 1 as suggested by you - blue vitriol is Copper sulphate penta hydrate and green vitriol is Ferrous sulphate hexahydrate, their molecular composition is not the same. There is a difference in water of crystallisation. Thus they are not isomorphic compounds.

Regards,