The four devices split into two emitters and two detectors, and within each pair one clear property separates them — giving a-iv, b-iii, c-ii, d-i, option 3.

LASER against LED — the emitters. Both work by recombination in a forward-biased junction, but the LED emits spontaneously: each photon is released independently, in a random direction and with a random phase, giving a broad spectrum some tens of nanometres wide and a diffuse beam. The laser adds an optical cavity and enough pumping to reach population inversion, so emission becomes stimulated — each photon triggers another identical to it in phase and direction. The result is coherent, far narrower in linewidth, and enormously brighter for the same junction area.

Solar cell against photodiode — the detectors. Physically they are the same p-n junction; the difference is the quadrant of the I-V characteristic in which they are operated.

The solar cell works with no external bias, in the fourth quadrant where the voltage is positive and the current negative. The product \(VI\) is therefore negative — the junction is a source, and it delivers power to the load. That is its whole purpose, characterised by

\(P_{max}=V_{oc}I_{sc}\times FF\)

The photodiode is reverse biased, in the third quadrant, so voltage and current have the same sign and the device consumes power from the supply. Nothing is gained in energy — what is gained is speed and linearity: the reverse bias widens the depletion region, reduces the junction capacitance, and makes the photocurrent very accurately proportional to the incident light.

The pairing of purposes is neat: energy conversion wants the unbiased mode, and measurement wants the biased one.

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