Monochromatic radiation of wavelength λ is incident on a hydrogen...

As $H$-atom emits $6$ spectral lines $\dfrac{n\left(n - 1\right)}{2} = 6$

$∴ n = 4$

$\Delta E = {E}_{4} - {E}_{1} = 13. 6 - 0. 85$

$= 12. 75eV$

$\lambda = \dfrac{hc}{∆ E} = \dfrac{1242nm}{12. 75}$

$∴ \lambda = 97. 5nm$