∵ For bcc lattice, radius ratio for r+∕r−is 0.7, i.e. radius ratio of cation by anion ranges from 0.732 to 1.0. Also, for bcc r−ions are at corners, Thus, ‌‌4×r−=√3a‌‌(∵√3a=1.7×200) (where, a= edgelength ) and 4×r−=1.7×200 r−=‌
1.7×200
4
=85pm ∴‌‌‌
r+
r−
=‌
70
85
=0.82, which is close to 0.7. Hence, option (a) is the correct answer.