Particles in a gas are best described as

Slow - moving, kinetic, hard spheres

Spheres that are in fixed positions when trapped in a container

Small, hard spheres with insignificant volumes

Hard spheres influenced by repulsive forces from other spheres

I think the question here would be related to the kinetic molecular theory of gases. So, the answer would be that particles in a gas are best described as small, hard spheres with insignificant volumes. It is the only choice that is one of the assumption of such theory.