Answer
The atoms in hydrogen gas at room temperature are almost all in the ground state of n=1.
Work Step by Step
When hydrogen gas is at room temperature, nearly all the H atoms are in the n=1 ground state. When a wide spectrum of light passes through the gas, photons are absorbed that are found in the Lyman series because these transitions involve electrons coming from or dropping to the n=1 ground state.
Other series, containing photons that correspond to transitions between higher n states, are not absorbed because almost no atoms start in higher energy states.
