The best gating signal for a single-phase full-wave regulator feeding an R-L load is a pulse train.

Explanation:

Single-phase Full Wave Regulator
A single-phase full-wave regulator is a power electronic device used to control the output voltage of an AC power supply. It consists of a rectifier circuit followed by a regulator circuit. The rectifier circuit converts the AC input voltage into a pulsating DC voltage, and the regulator circuit controls the amplitude of the DC voltage to obtain the desired output voltage.

R-L Load
An R-L load refers to a load that consists of a resistor (R) and an inductor (L) connected in series. This type of load is commonly encountered in many electrical systems and devices.

Gating Signal
The gating signal is the control signal that determines the conduction period of the power electronic device. In the case of a single-phase full-wave regulator, the gating signal controls the conduction of the rectifier circuit.

Short Duration Pulses vs. Long Duration Pulses
Short duration pulses refer to pulses with a narrow width, while long duration pulses refer to pulses with a wide width. The duration of the gating pulses affects the conduction period of the rectifier circuit.

Pulse Train
A pulse train refers to a sequence of pulses that are regularly spaced and have a consistent width. In the context of a single-phase full-wave regulator, a pulse train gating signal is preferred because it allows for continuous control of the output voltage.

Advantages of Pulse Train Gating Signal
1. Continuous Control: A pulse train gating signal allows for continuous control of the output voltage. By adjusting the duty cycle (on-time/off-time ratio) of the pulses, the average output voltage can be varied smoothly.
2. Reduced Ripple: A pulse train gating signal helps in reducing the output voltage ripple. The regular spacing of the pulses ensures a more uniform output voltage waveform.
3. Improved Efficiency: By using a pulse train gating signal, the conduction losses in the rectifier circuit can be minimized, resulting in improved overall efficiency of the system.

Conclusion
In conclusion, the best gating signal for a single-phase full-wave regulator feeding an R-L load is a pulse train. It allows for continuous control of the output voltage, reduces ripple, and improves system efficiency. Short duration or long duration pulses are not as effective in achieving these objectives.