Answer:
The triggering methods for series connected SCRs are used to turn on or trigger the SCRs in a controlled manner. These methods include resistance triggering, simultaneous triggering, sequential triggering, and optical triggering. Among these methods, the LASCR (Light Activated Silicon Controlled Rectifier) is used in the gate circuit for optical triggering.

Optical Triggering:
Optical triggering is a method of triggering SCRs using light. In this method, a light source is used to illuminate a photosensitive device, which generates a voltage or current that is used to trigger the gate of the SCR. The LASCR is a type of SCR that is designed to be triggered by light.

Working Principle of LASCR:
The LASCR has a gate structure that is sensitive to light. When light falls on the gate structure, it generates a current that can be used to trigger the SCR. The gate structure of the LASCR consists of a photosensitive material, typically a silicon wafer with a p-n junction. When light falls on the photosensitive material, it excites the electrons, creating a flow of current.

Advantages of Optical Triggering:
- Optical triggering provides isolation between the control circuit and the power circuit, as no electrical connection is required between them.
- It allows for precise control and fast switching of the SCRs.
- Optical triggering is immune to electrical noise and interference.

Other Triggering Methods:
1. Resistance Triggering:
- In resistance triggering, a resistor is connected in series with the gate of the SCR.
- A voltage is applied across the resistor, causing a current to flow through it and trigger the SCR.

2. Simultaneous Triggering:
- In simultaneous triggering, a single trigger pulse is applied to the gates of all the SCRs in the series-connected circuit.
- This causes all the SCRs to turn on simultaneously.

3. Sequential Triggering:
- In sequential triggering, individual trigger pulses are applied to the gates of each SCR in a specific sequence.
- This allows for controlled switching of the SCRs one after the other.

Conclusion:
Among the given triggering methods, only the optical triggering method uses LASCR in the gate circuit. LASCR is designed to be triggered by light, and it provides advantages such as isolation, precise control, and immunity to electrical noise. The other triggering methods involve resistance, simultaneous pulses, or sequential pulses to trigger the SCRs.