The list is already in its natural order: a-i, b-ii, c-iii, d-iv — option 1.

c is the surest anchor. A folded dipole's two parallel conductors carry equal in-phase currents, so for the same radiated power the current in the feed arm is halved; since \(P=I^{2}R\), the feed-point resistance is multiplied by four:

\(R_{folded}=4\times73\approx292\approx300\ \Omega\)

That is why 300 Ω ribbon feeder exists, and why the folded dipole is the driven element of almost every Yagi television aerial — the parasitic elements pull the impedance down, and starting from 300 Ω leaves it near a usable value.

a — the loop's null. A small loop responds to the magnetic field threading it, so the induced EMF is proportional to \(\cos\theta\) measured from the loop's plane. Along the axis — broadside to the loop — no flux links it at all and the response falls to zero. The null is sharp, far sharper than the broad maximum, which is exactly why direction finding uses the null rather than the peak to take a bearing.

b — the rhombic. Four wires several wavelengths long, arranged as a diamond and terminated in a resistor at the far apex. The termination absorbs the reverse wave, making it a travelling-wave antenna: unidirectional, high gain, and usefully broadband, which is why rhombics were the standard for long-distance HF point-to-point links.

d — polarisation. The polarisation of a wire antenna is the direction of its electric field, which lies along the conductor. A horizontal wire radiates horizontal polarisation; a vertical one, vertical. Matching the two ends matters — a cross-polarised receiving antenna loses 20 dB or more.

Hence, the correct code is a-i, b-ii, c-iii, d-iv.