A fiber optic cable transmits information as pulses of light rather than as electrical current. Its core is an extremely thin strand of highly pure glass (or plastic), surrounded by a cladding layer of slightly lower refractive index. Because the core index is higher than the cladding index, a light ray entering at a suitable angle undergoes total internal reflection at the core–cladding boundary and is guided along the fiber with very little loss, even around gentle bends.

At the transmitting end, an LED or laser diode converts electrical data into light; at the receiving end, a photodiode converts the light back into an electrical signal. This optical mechanism gives fiber several important advantages:

• Immunity to electromagnetic interference (EMI), since light is unaffected by external electric or magnetic fields.
• Very high bandwidth and low attenuation, allowing high data rates over long distances without frequent repeaters.
• No electrical crosstalk and enhanced signal security.

The other options all carry electrical signals. Twisted pair cable uses pairs of copper wires and relies on twisting to cancel noise. Coaxial cable uses a central copper conductor with a shield and passes electrical signals for cable TV and older networks. A USB cable likewise carries electrical signals over copper conductors for data and power. Only the fiber optic cable uses light, so it is the correct answer.