The correct option is B Zn2+ concentration should be decreased
Zn(s)+Cl2(1 atm)→Zn2++2Cl−
The EMF of the cell is
Ecell=E∘cell−RTnFln[Zn2+][Cl−]2PCl2
To increase the Ecell, the ln[Zn2+][Cl−]2PCl2 term should decrease. So Zn2+ concentration should be decreased and partial pressure of Cl2 should increased,