The assertion is plainly true; the reason, read charitably, is also true — but it explains nothing about bandwidth — option 2.

Why fibre has more bandwidth, and it is not about laser power. The usable bandwidth of any carrier system is a fraction of its carrier frequency. Optical carriers run at

\(f=\dfrac{c}{\lambda}=\dfrac{3\times10^{8}}{1.55\times10^{-6}}\approx193\ \text{THz}\)

against a few hundred megahertz for coaxial cable — six orders of magnitude. Even one per cent of that carrier is terahertz of bandwidth, which is why a single fibre carries tens of terabits per second using wavelength-division multiplexing.

The second reason copper loses is that its loss rises steeply with frequency, because of the skin effect and dielectric absorption; a coaxial cable useful to 500 MHz over a kilometre is useless at 5 GHz. Fibre attenuation, about 0.2 dB/km, is essentially flat across its whole transmission window, so raising the data rate costs nothing in loss. Neither of these arguments involves optical power at all.

What the reason is getting at. A laser's output is coherent, monochromatic and highly directional, so its irradiance — power per unit area per unit bandwidth — hugely exceeds that of a lamp radiating a broad spectrum into every direction, even one drawing far more electrical power. In that restricted sense the statement is true, and it does explain why a laser can be coupled efficiently into a 9 µm core while a lamp cannot.

But that is a statement about coupling and range, not bandwidth. Power determines how far a signal reaches before it needs regenerating; bandwidth is set by carrier frequency and by dispersion. A high-power source down a copper cable does not give it terahertz bandwidth, and a low-power source down a fibre does not take it away.

The answer is flagged because R is loosely worded and could be judged false outright; on either reading, however, it is not an explanation of A.

Hence, both are correct, but (R) is not the explanation of (A).