A signal of $$100$$ THz frequency can be transmitted with maximum efficiency by

The frequency of the signal is $$100$$ THz $$= 100 \times 10^{12}$$ Hz $$= 10^{14}$$ Hz.

This frequency falls in the infrared/optical range of the electromagnetic spectrum.

Let us examine each transmission medium:

Coaxial cable: Suitable for frequencies up to about $$1$$ GHz ($$10^9$$ Hz). Cannot handle $$10^{14}$$ Hz signals efficiently.

Optical fibre: Designed to transmit signals in the optical and near-infrared frequency range ($$10^{13}$$ to $$10^{15}$$ Hz). This is the ideal medium for $$100$$ THz signals.

Twisted pair of copper wires: Suitable for frequencies up to a few MHz. Completely inadequate for $$10^{14}$$ Hz.

Water: Not a practical transmission medium for electromagnetic signals at this frequency due to high absorption.

Since $$100$$ THz lies in the optical frequency range, optical fibre transmits this signal with maximum efficiency through the principle of total internal reflection.

The correct answer is Option B.