Bandwidth follows carrier frequency, so ranking the media by the frequencies they work at ranks them by capacity.

Descending order: fibre, microwave, coax, twisted pair — (d), (b), (c), (a), which is option 2.

Optical fibre is in a class of its own. Its carrier frequency is around \(2\times10^{14}\) Hz, and even 1 % of that is 2 THz. In practice the low-loss windows around 1310 and 1550 nm offer tens of terahertz of usable spectrum, exploited by wavelength-division multiplexing to carry many independent channels down one strand.

Microwave beats coaxial cable because it is not limited by a conductor at all. A coaxial cable's usable band is capped by skin-effect attenuation, which grows as \(\sqrt{f}\) — beyond about a gigahertz the loss per metre becomes prohibitive. A microwave radio link suffers no such conductor loss and routinely operates from a few GHz to tens of GHz, so its available bandwidth is larger. This is the one comparison in the list that is not obvious at a glance, and it is why the answer is flagged for confirmation.

Twisted pair is last for two reasons: the pair radiates and picks up interference as frequency rises, and the conductor loss again climbs with the square root of frequency. Category cabling pushes it to 100 MHz (Cat 5e), 250 MHz (Cat 6) and 500 MHz (Cat 6a), but only over short runs.

The general principle behind the whole ranking is that a modulated channel's bandwidth is bounded by a fraction of its carrier frequency, so the higher the carrier, the more room there is. Every step up the list — copper to radio to light — is a step up in carrier frequency of three or more orders of magnitude, and capacity follows.

Hence, the descending order is (d), (b), (c), (a).