The correct answer is (b) Silicon.
Silicon is the most commonly used material for making transistors because it is a semiconductor. Semiconductors are materials that have electrical conductivity between that of a conductor and an insulator. This makes them ideal for use in transistors, which are devices that control the flow of electricity.
Aluminium, copper, and silver are all conductors. Conductors are materials that allow electricity to flow easily through them. This makes them unsuitable for use in transistors, which need to be able to control the flow of electricity.
Silicon is a relatively abundant element, which makes it inexpensive to produce. It is also a relatively stable element, which means that it does not corrode easily. This makes it a good choice for use in transistors, which need to be able to withstand harsh environmental conditions.
Silicon is also a relatively easy material to work with. It can be easily shaped and molded into the desired form. This makes it a good choice for use in transistors, which need to be able to be manufactured in large quantities.
In conclusion, silicon is the most commonly used material for making transistors because it is a semiconductor, it is relatively inexpensive, it is relatively stable, and it is relatively easy to work with.