The correct answer is option 'C': the gate pulses should be rectangular with a width of (p - a) in a single-phase full wave ac regulator feeding an R-L load. Let's understand why this is the case:

1. Single-phase full wave ac regulator:
A single-phase full wave ac regulator is a power electronic device used to control the voltage or power delivered to an R-L load. It consists of a thyristor (SCR) connected in series with the load, and the gate of the thyristor is controlled to regulate the output voltage or power.

2. Gate pulses:
Gate pulses are applied to the gate terminal of the thyristor to trigger it into conduction. By controlling the timing and duration of the gate pulses, the average voltage across the load can be varied, thereby regulating the output voltage or power.

3. Rectangular pulses:
To turn on the thyristor, a gate pulse with sufficient amplitude and duration is required. The gate pulse should have a rectangular shape to ensure that the thyristor turns on quickly and reliably. The rectangular shape allows the thyristor to transition from the non-conducting state to the conducting state effectively.

4. Width of (p - a):
The width of the gate pulse should be (p - a), where p is the period of the input ac waveform and a is the delay angle. The delay angle represents the time delay between the zero crossing of the input waveform and the initiation of the gate pulse.

5. Reason for (p - a) width:
The width of (p - a) ensures that the thyristor turns on at the desired point in the input waveform. If the gate pulse is too short, the thyristor may not turn on properly, leading to a distorted output waveform. If the gate pulse is too long, the thyristor may turn on too early, causing a higher average voltage across the load.

In conclusion, the gate pulses in a single-phase full wave ac regulator feeding an R-L load should be rectangular with a width of (p - a). This ensures proper triggering of the thyristor and accurate regulation of the output voltage or power.