The correct option is C 9r216
Bohr's radius for nth orbit, (rn)=0.529[n2Z]∘A
where n=Orbit number, Z=Atomic number
(rn)=r2 for 2nd orbit of hydrogen
Therefore, ratio of both the radius gives:
rHrBe3+=n2HZH×ZBe3+n2Be3+=22×41×32
⇒r2rBe3+=169
⇒rBe3+=9r216