To me it looks like gas.
Gas- CORRECT!
Solid- incorrect because if this were a depiction of a solid then the particles would be much much closer together.
Liquid - incorrect because the particles would be a little more crowded together.
Plasma- I do not think that this answer is correct because, even though the spacing of particles is similar to the spacing of particles in plasma, the particles in plasma are charged particles(nuclei (+) and electrons(-)). Therefore, because the diagram doesn't have the particles labeled with charges, I believe that Plasma cannot be the correct answer.
Any questions?