The helical antenna in axial mode radiates a circularly polarised wave — option 4.

Why the geometry produces circular polarisation. Current travelling round a helical turn has both an axial component and a circumferential one. When the circumference of a turn is about one wavelength,

\(C\approx\lambda \quad\Longrightarrow\quad \text{one full turn} = \text{one period}\)

the current takes exactly one RF cycle to travel round a turn. The two orthogonal field components therefore emerge equal in magnitude and 90° apart in phase — which is precisely the definition of circular polarisation:

\(\vec{E}=E_0\left(\hat{a}_x\pm j\hat{a}_y\right)e^{-jkz}\)

The sense follows the winding: a right-hand-wound helix radiates right-hand circular, and it must be received by a right-hand antenna, since the opposite sense is cross-polarised and rejected.

Why the other three are linearly polarised.

The small loop is the deliberate trap: circular geometry does not imply circular polarisation. Polarisation describes the behaviour of the electric field vector in time, not the outline of the conductor.

The general condition for circular polarisation. Two orthogonal field components of equal amplitude with a 90° phase difference. Unequal amplitudes give elliptical polarisation; zero phase difference gives linear. This is why the other common way of generating it is to feed crossed dipoles through a quarter-wavelength delay.

Why it is worth the trouble. A circularly polarised wave has no fixed plane, so it is immune to Faraday rotation in the ionosphere and to the unknown orientation of a moving or tumbling terminal. That is why satellite links, GPS and telemetry use it — and why the helix, with its wide bandwidth of about 1.7:1 and near-resistive 140 Ω input, became the standard tracking feed.

Hence, the circularly polarised antenna is the helical.