For a coaxial cable, the ___ serves as a conductor.

In a coaxial cable, the center conductor is the primary component that serves as the conductor for transmitting signals. This center conductor is typically made of copper or aluminum and is responsible for carrying the electrical signals. Surrounding the center conductor is a layer of dielectric material, which insulates it from the outer layer.

The shielding, which usually consists of a metal braid or foil, plays a crucial role, but it primarily serves as a ground reference and provides protection against electromagnetic interference. While both the center conductor and the shielding can carry signals under specific conditions, the primary function of signal conduction is performed by the center conductor.

Therefore, when considering the question, the center conductor is the key component that directly serves as the conductor for signal transmission, while the shielding has supporting roles such as providing electrical grounding and enhancing signal quality. This makes the answer that acknowledges both components as conductors appropriate, which is why "both a and b" is deemed the correct choice.