(a) — UJT as the phase-control device ✓. A unijunction transistor in a relaxation oscillator is the classic SCR trigger circuit: a capacitor charges through a variable resistor until it reaches the UJT's peak-point voltage \(V_P=\eta V_{BB}+V_D\), at which point the UJT fires and dumps a sharp pulse into the SCR gate. Adjusting the RC time constant moves the firing instant within each half cycle — that is exactly phase control, and the pulse is naturally short, which suits gate drive.

(c) — maximum power at zero firing angle ✓. For a half-wave controlled rectifier

\(V_{dc}=\dfrac{V_m}{2\pi}(1+\cos\alpha)\)

which is largest at α = 0 and falls to zero at α = 180°. Firing at zero delay makes the SCR conduct for the whole available half cycle, so it behaves like an ordinary diode and delivers maximum power.

(d) — on/off switching thousands of times a second ✓. An SCR fires once per cycle of the supply, so on a 50 Hz mains it switches 50–100 times a second; in inverter and chopper circuits with forced commutation the switching rate reaches many kilohertz. Control by switching rather than by dropping voltage across a series element is what makes thyristor control so efficient — the device is either fully on or fully off, so it dissipates very little.

(b) — SCR rectifier gives a greater average voltage than a diode rectifier ✗. Exactly the reverse. The whole purpose of the firing angle is to delay conduction, so part of each half cycle is lost. The best an SCR can do is α = 0, where it merely equals the diode; for every α > 0 the average output is lower. This is the statement the question turns on.

So the true statements are (a), (c) and (d) — option 2.

Hence, the correct answer is (a), (c) and (d).