The correct answer is: D. holes
A hole is a vacancy in the valence band of a semiconductor. It can be thought of as a positively charged particle that can move through the material. Holes are the majority carriers in p-type semiconductors.
In a pnp transistor, the current is carried by holes in the p-type base region. The holes are injected into the base region by the emitter, and they then diffuse into the collector region. The collector region is heavily doped with n-type material, so there are many free electrons available to combine with the holes. This combination of holes and electrons produces a current that flows through the collector circuit.
Acceptor ions are negatively charged ions that are formed when a semiconductor is doped with p-type material. Donor ions are positively charged ions that are formed when a semiconductor is doped with n-type material. Free electrons are electrons that are not bound to atoms in a semiconductor.
I hope this explanation is helpful!