a b c d

d

$\text{Total number of radiation emitted when an electron gets deexcited to ground level is given by} \frac{n(n-1)}{2} = 6$ $\Rightarrow n^2 - n -12 = 0 , \Rightarrow n = 4$ $\Rightarrow \frac{1}{\lambda} = R\times 3^2(\frac{1}{1^2} - \frac{1}{4^2} )= 9R\times \frac{15}{16}$ $\Rightarrow \lambda = \frac{16}{135R} = \frac{16\times 91.2nm}{135} = 10.8nm$