The correct option is (D) Li7.
Explanation:
R = Ro ∙ A(1/3) ------ Bohr's radius
when A = 189 then R = Ro A(1/3) i.e. R = Ro(189)(1/3) (1)
when R = (R / 3) then (R / 3) = Ro(A)(1/3) (2)
∴ from (1) & (2)
[R / (R / 3)] = [(189) / A](1/3)
∴ (3)3 = [(189) / A]
∴ A = [(189) / (27)] = 7
∴ nucleus is Li7