Wood, plastic, rubber and air are good insulators because
a. they conduct charge.
b. charge flows easily.
c. they don’t have free electrons.
d. they have low resistance.

c) they don’t have free electrons.

Explanation:  Wood, plastic, rubber, and air are good insulators because, obviously, they are bad conductors.

This is because the do not have a lot of free electrons, so when you apply an electric field, the total flow of electrons (or electricity) is almost zero.

In the same case, when charges try to flow into these materials (by connecting a wire to a piece of wood or something like that), there are no free electrons inside of the material, so the electrons that try to move inside the material see a lot more of obstacles against their movement.

Then the correct option is option c: they don’t have free electrons.

asthenosphere and convection

c. magnitude and direction