Two pairings are textbook definitions and they carry the rest: a-iii, b-i, c-iv, d-ii — option 1.

a — what the snubber protects against. A thyristor's internal junctions have capacitance. A rapidly rising anode voltage drives a displacement current through it,

\(i=C_{j}\dfrac{dv}{dt}\)

and if that current is large enough it acts like gate current and turns the device on when no trigger was applied — a false firing that can short the supply. An RC snubber across the device gives that current a path through the capacitor instead, slowing the rate of rise the junction sees; the series resistor limits the discharge current when the device does turn on legitimately.

b — the inverter and the SCR. An inverter turns DC into AC by switching, and thyristors are the classic switching element for high-power inverters. The difficulty is that an SCR on a DC supply has no natural current zero at which to turn off, so forced commutation is needed — an auxiliary LC circuit that drives the anode current briefly to zero. That complication is the whole reason GTOs and IGBTs displaced SCRs in medium-power inverters.

c — the UJT and phase control. Controlling AC power by delaying the firing angle needs a pulse at a precisely adjustable point in each half cycle. A unijunction transistor relaxation oscillator produces exactly that: a capacitor charges through a variable resistor until the emitter reaches the peak point,

\(V_{P}=\eta V_{BB}+V_{D}\)

whereupon the UJT fires and dumps the charge into the thyristor's gate as a sharp pulse. Turning the resistor moves the firing angle and so the delivered power.

d — why a switched-mode supply is efficient. A linear regulator's pass transistor is in the active region continuously, dropping the excess voltage and dissipating it as heat — efficiency of perhaps 40 %. An SMPS switches its transistor fully on or fully off. When on, the voltage across it is nearly zero; when off, the current through it is nearly zero; and power is the product, so dissipation is small in both states. Efficiencies of 85 to 95 % follow, and the high switching frequency also allows a much smaller transformer.

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