∵ In sodium chloride crystal, the distance between Na+ and Cl− is Ypm. ∵NaCl has fec structure in which Na+ occupies all octahedral voids, while Cl− ions are present at all corners and edge centers. If, edge length =a The distance between Na+ and Cl−=Ypm (Given) a=2(rCI−+rNa+)=2Ypm Hence, edge length is 2Y pm and option (b) is the correct answer.