Explain why a single atom of hydrogen cannot produce all four hydrogen spectral lines simultaneously.

When the electron in a hydrogen atom transitions from a high energy state to a lower energy state, the energy lost from the electron is used to produce a photon corresponding to the loss of energy. That photon will correspond to exactly 1 wavelength. And since a hydrogen atom has only 1 electron, at any given moment, it can only produce 1 photon. And in order to simultaneously produce 4 photons for 4 spectral lines, that would require a simultaneous transition of 4 electrons which is 3 too many for a hydrogen atom.

It shows that the experiment is designed properly.