In a PNP transistor operating in the active region, the main stream of current is the diffusion of holes. Let's understand why this is the correct answer by breaking it down into key points.

1. PNP Transistor: A PNP transistor is a type of bipolar junction transistor (BJT). It consists of three layers of semiconductor material: P-doped (positive) layer, N-doped (negative) layer, and another P-doped layer. The middle N-doped layer is called the base, and the outer P-doped layers are called the emitter and collector.

2. Active Region: The active region is the operating state of the transistor where it is used as an amplifier. In this region, both junctions (base-emitter and base-collector) are forward-biased. The base-emitter junction is biased with a higher voltage than the base-collector junction.

3. Current Flow: In the active region, the main stream of current is the movement of majority charge carriers. In a PNP transistor, the majority charge carriers are holes (positive charge carriers) in the P-doped regions and electrons (negative charge carriers) in the N-doped region.

4. Diffusion of Holes: When the base-emitter junction is forward-biased, a potential barrier is lowered, allowing the flow of majority charge carriers across the junction. In the case of a PNP transistor, holes from the base region diffuse into the emitter region.

5. Electron Flow: While there is some electron flow in the transistor, it is not the main stream of current in the active region. Electrons from the emitter region do diffuse into the base region, but they are minority charge carriers and are not the dominant current carriers.

6. Drift of Holes: The drift of holes refers to the movement of holes due to the presence of an electric field. However, in the active region of a PNP transistor, the dominant current flow is due to the diffusion of holes, not their drift under the electric field.

Therefore, the correct answer is the diffusion of holes (option C). It is important to understand the principle of majority and minority charge carriers and their behavior in different regions of a transistor to comprehend the current flow in various transistor configurations.