The correct answer is C. One.
A depletion layer is a region in a semiconductor where there are no free charge carriers. It is formed at the junction of two semiconductor materials with different electrical properties. In a transistor, there are two depletion layers, one at the base-emitter junction and one at the base-collector junction.
The depletion layer at the base-emitter junction is formed when electrons from the emitter diffuse into the base. This diffusion of electrons creates a region of positive charge in the emitter and a region of negative charge in the base. The depletion layer at the base-collector junction is formed when electrons from the base diffuse into the collector. This diffusion of electrons creates a region of positive charge in the base and a region of negative charge in the collector.
The depletion layers in a transistor play an important role in the operation of the transistor. They control the flow of current through the transistor and determine the transistor’s gain.
Option A is incorrect because there are two depletion layers in a transistor, not four.
Option B is incorrect because there are two depletion layers in a transistor, not three.
Option D is incorrect because there are two depletion layers in a transistor, not one.