According to Bohr's theory of hydrogen atom., the product of the binding energy of the electron in the $nth$ orbit and its radius in the nth orbit.
(a) is proportional to $n$
(b) is inverscly proportional to $n^{4}$
(c) has a constant valuc $7 \cdot 2 eV$
(d) has a constant value of $13 \cdot 6 eV$