A diode laser (answer (A)) and a solid-state laser (answer (E)) are the same thing. They use transitions from the conduction band to the valence band of a semiconductor. So, answers (A) and (E) are not correct. A free-electron laser (answer (C)) takes advantage of the fact that electrons radiate when they are accelerated or decelerated. The electrons are not contained in atoms, so answer (C) is not correct. In a dye laser (answer (B)), there are also no free atoms and the lasing transitions are transitions between different molecular states, not different atomic states. A gas laser (answer (D)) does indeed involve free atoms and the lasing transition is a transition between two energy levels of a free atom. For example, in a helium-neon laser, there are free helium and neon atoms, and the lasing transition is a transition between two states in the neon atom. Therefore, answer (D) is correct.