Which reason best explains why plasmas are good conductors of electricity? they have mobile charged particles. they are at extremely high temperatures. they are made up of particles in fixed positions. they have slow-moving nuclei that carry electricity.

All dielectrics have a certain value of breakdown potential. In most of them like air, ceramics, etc. (including semiconductors like Silicon), this potential provides sufficient energy to ionize a few atoms. The free electrons formed as a result of this ionization are energized enough to bump the neighborhood atoms, break some covalent bonds and produce more free electrons.

These free electrons are then accelerated by the applied electric field and they collide and ionize the other atoms to produce more free electrons (multiplication by collision). Now, there are large number of free electrons available for current flow. Thus, an electric arc is produced!

However, this isn't applicable for all gases., there are a few other cases such as the noble gases which are mono-atomic (and are used in most of the discharge tubes), whose breakdown potentials are only meant to ionize them!

They have mobile charged particles.