The correct option is A 3
nth excited state means that the electron is in the (n+1)th energy level. Given, the sample of these 4−H atoms shows a total of six different spectral lines, while returning to the ground level, that is possible only when (n+1)=4. The respective transitions will be 4→1, 4→2, 4→3, 3→2, 3→1, 2→1
Hence, option (a) is correct.