The correct answer is: C. both a.c. and d.c.
Transistor biasing is the process of setting the operating point of a transistor. This is done by applying a small amount of DC voltage to the base-emitter junction. The DC voltage causes the transistor to operate in a linear region, where it can amplify signals.
The AC signal is then applied to the base-emitter junction. The AC signal causes the transistor to switch between its on and off states. This switching action is what allows the transistor to amplify signals.
Therefore, transistor biasing represents both AC and DC conditions.
Option A is incorrect because transistor biasing does not represent AC conditions only.
Option B is incorrect because transistor biasing does not represent DC conditions only.
Option D is incorrect because transistor biasing represents both AC and DC conditions.