A hydrogen atom in its ground state absorbs a photon and is excited to $n = 4$. The number of *possible* spectral lines in its subsequent emission is

From $n=4$, electrons can drop to $n=3, 2, 1$. Distinct lines $= \binom{4}{2} = 6$ — covering Lyman ($\to 1$), Balmer ($\to 2$) and Paschen ($\to 3$) series transitions.