Hydrogen atom in ground state is excited by a monochromatic radiation of λ=975oA. Number of spectral lines in the resulting spectrum emitted will be

Energy of the photon, E=hc/λ=1240/97.5=12.75eV
This energy is equal to energy gap between n=1 (13.6) and n=4 (0.85). So by this energy, the electron will excite from n=1 to n=4.
When the electron will fall back, numbers of spectral lines emitted = n(n-1)/2 = (4)(4-1)/2 = 6