Energy is related to mass
$E_n\propto m$
The longest wavelength $\lambda_{max}$ photon will correspond to the transition of particle from n=3 to n=2
$\large\frac{1}{\lambda_{max}} = 2R(\large\frac{1}{2^2}-\large\frac{1}{3^2})$
$\lambda_{max} = \large\frac{18}{5R}$
Hence answer is (c)