Zn rod is placed in 100ml of 1M CuSO₄ solution so that molarity of Cu⁺² changes to 0.7M . the molarity of SO₄-- at this stage will be

1) 0.8M2) 1M3) 0.7M4)1.8M

The correct option is b)
The molarity of sulphate ion would be same that is 1M as it does not participate in reaction. The reaction is as follows:
Zn + Cu²⁺ $\to$Zn²⁺ + Cu
The sulphate ion would be same as before and after the reaction.