The correct option is B n = 2
For an atom of atomic number Z, the radius of the nth orbit is given by rn=Kn2Z where K is a constant
For the ground state of hydrogen, Z=1, n=1 and for Be3+, Z=4
By the given condition, n21Z1=n22Z2
⇒11=n224
⇒n2=2
Hence, the correct choice is (b).