Three photons coming from excited atomic hydrogen sample are picked up. Their energies are 12.1eV, 10.2 eV, and 1.9 eV. These photons must come from

All these wavelength lie in the spectrum of the hydrogen atom for the spectral lines corresponding to the transitions 3→1,2→1,and3→2. 

Therefore, minimum 2 atoms are required with one going through 3→1 transition and the other going through   3→2→1 transition.