The correct answer is B. Silicon.
A transistor is a semiconductor device used to amplify and switch electronic signals and electrical power. It is composed of semiconductor material with at least three terminals for connection to an external circuit. A voltage or current applied to one pair of the transistor’s terminals changes the current through another pair of terminals. Because the controlled (output) power can be higher than the controlling (input) power, a transistor can amplify a signal. Today, some transistors are packaged individually, but many more are found embedded in integrated circuits.
The BC147 is a small-signal transistor made of silicon. It is a NPN transistor, which means that it has three terminals: the base, the collector, and the emitter. The base is the control terminal, and the collector and emitter are the output terminals. When a small voltage is applied to the base, it controls a much larger current that flows between the collector and emitter. This makes the BC147 useful for amplifying signals.
The other options are incorrect. Germanium is a semiconductor material that was used in early transistors, but it has been largely replaced by silicon. Carbon is not a semiconductor material, and it is not used in transistors.