The principle that decides every case. An antenna is broadband when its behaviour is governed by angles rather than by a fixed length. If the geometry can be described by angles alone, the structure looks electrically similar at every frequency and the impedance stays roughly constant. If instead the antenna is resonant — defined by a length that must equal a particular fraction of a wavelength — it works only near that one frequency.

Discone — wideband. A disc above a cone is a classic frequency-independent structure: the cone's flare is set by an angle, and the cone presents a progressively larger effective aperture as the wavelength grows. Bandwidths of 10:1 in impedance are routine, which is why discones are the standard antenna on wideband scanning receivers.

Helical — wideband. In axial mode the helix radiates a circularly polarised beam along its axis over roughly a 1.7:1 frequency range, with an almost purely resistive input impedance near 140 Ω. It is the standard antenna for satellite links, where the wide band and circular polarisation both matter.

Folded dipole — relatively wideband. Two parallel conductors joined at the ends behave like a thick radiator; the increased effective diameter lowers the Q of the resonance and flattens the impedance curve, giving several times the bandwidth of a plain dipole. The folding also raises the radiation resistance from 73 Ω to about 292 Ω, which is why it matches 300 Ω twin feeder directly — the familiar television and FM receiving antenna.

Marconi — narrowband, so this is the answer. The Marconi antenna is a quarter-wave vertical monopole worked against a ground plane, the image in the ground completing the equivalent half-wave dipole. Its very definition is a length:

\(L=\dfrac{\lambda}{4}\)

so it is resonant at one frequency, with radiation resistance about 36.5 Ω and a sharply rising reactance either side of resonance. Change the frequency and the antenna is simply the wrong length. It is used for AM broadcast and other long-wave services, where a single assigned channel is all that is needed — the passage's mention of AM transmission is the clue.

The test in one line. Discone, helix and folded dipole all achieve bandwidth through geometry — flare angle, pitch angle, or effective thickness. The Marconi has none of these; it has only a length.

Hence, the antenna that is not wideband is the Marconi.