The correct option is C n=2,Be3+
Radius (rn)=n2a0Z
For Bohr's orbit of H-atom, Z=1, n=1
∴r1=a0
(a) n=3,Li2+(Z=3) r2=9a03=3a0(b) n=2,He+(Z=2) r2=4a02=2a0(c) n=2,Be3+(Z=4) r2=4a04=a0(d) n=2,Li3+(Z=3) r2=4a03
So radius of second orbit (n=2) for Be3+ has the same radius as that of Bohr's first orbit of H-atom