(d) Number of wavelength $= \frac{n(n - 1)}{2}$, where $n =$ No. of orbit from which transition takes place
$\therefore 6 = \frac{n(n - 1)}{2} \Rightarrow n = 4$
In all given options wavelength of emitted radiation’s will be maximum for transition $n = 4$ to $n = 3$