Introduction:
A single pulse semi-converter is a type of power electronic circuit that converts AC voltage into DC voltage. It consists of two thyristors (SCRs) connected in an anti-parallel configuration. The thyristors are triggered to conduct during specific intervals of the AC input waveform, allowing the desired portion of the input voltage to be rectified.

Explanation:
The triggering circuit in a single pulse semi-converter is responsible for initiating the conduction of the thyristors at the appropriate time. The main objective of the triggering circuit is to produce a firing pulse for the thyristors.

Half Cycle Operation:
During each half cycle of the AC input waveform, one SCR is triggered to conduct while the other SCR remains non-conducting. This allows the current to flow through the load in one direction, resulting in a rectified output voltage.

Triggering Requirement:
To achieve this operation, the triggering circuit must produce one firing pulse in each half cycle. The firing pulse is applied to the gate terminal of the SCR that needs to be triggered. When the firing pulse is applied, the SCR turns on and starts conducting.

Reason for Single Firing Pulse:
The reason for having only one firing pulse in each half cycle is that the objective of the single pulse semi-converter is to rectify the input voltage in a single direction. If multiple firing pulses were provided in each half cycle, it would result in multiple conduction intervals for the thyristors, leading to bidirectional current flow and an undesired output.

Rectified Output:
By having one firing pulse in each half cycle, the converter ensures that only one SCR conducts during that interval, allowing the desired portion of the input waveform to be rectified. This results in a unidirectional current flow through the load, providing the desired DC voltage output.

Answer:
Therefore, in a single pulse semi-converter using two SCRs, the triggering circuit must produce one firing pulse in each half cycle to achieve the desired rectification of the AC input voltage. Option 'B', which states "one firing pulse in each half cycle," is the correct answer.