How to solve it?

Dear student, 
The given cristal structure is a body centred cubic structure.
The electrostatic force 
F=(1/4πε₀)×((q₁​​​​​​q₂)/r²)
due to one Cs⁺ ion is balanced by diagonally opposite Cs⁺ ion.
hence the net electrostatic force on Cl^- ion due to eight Cs⁺ ion is zero.
so the answer is option a.
Regards.