The pure semiconductor among the options given is "silicon". A semiconductor is a material that has an electrical conductivity between that of a conductor and an insulator. Silicon is a widely used semiconductor material in electronics due to its abundance and desirable electrical properties. Carbon is not a semiconductor, and transistors are electronic devices made using semiconductor materials, but they are not semiconductors themselves. Phosphorus is also not a pure semiconductor, but it can be used as a dopant to modify the electrical properties of a semiconductor material such as silicon.