The correct option is A n=2
The radius of ground state of hydrogen atom = 0.529 oA
Let the lone electron of the triply-ionized Be3+ correspond to the level n
From the given data, both the above radii are equal.
So,
0.529=0.529×n2Z
⇒0.529=0.529×n24
[atomic number of Be=4 (Z=4)]
∴n=2