**Explanation:**

To understand the method of triggering Silicon Controlled Rectifier (SCR), let's first understand the structure and operation of an SCR.

**Structure of SCR:**
An SCR consists of three layers of semiconductor material - P-N-P, where P is the anode, N is the cathode, and P is the gate. The anode is connected to the positive terminal of the power supply, and the cathode is connected to the load. The gate is used to trigger the SCR.

**Operation of SCR:**
An SCR acts as a switch that allows current to flow from the anode to the cathode when it is triggered. When a positive voltage is applied to the anode with respect to the cathode, the SCR remains in the off state. However, when a positive voltage is applied to the gate with respect to the cathode, the SCR turns on and allows current to flow.

Now, let's discuss the different options given in the question:

**a) Application of positive pulse at cathode gate:**
When a positive pulse is applied to the gate with respect to the cathode, it causes a flow of positive charge carriers towards the gate region. This creates a region of low resistance in the gate region, allowing the gate current to flow. This gate current triggers the SCR and turns it on. Therefore, applying a positive pulse at the cathode gate is the correct method of triggering an SCR.

**b) Application of pulse at anode gate:**
Applying a pulse at the anode gate does not trigger the SCR because the gate is not connected to the anode. The gate needs to be triggered with respect to the cathode, not the anode.

**c) Application of pulse at cathode gate:**
This option is similar to option a, where the pulse is applied at the cathode gate. However, it does not specify whether the pulse is positive or negative. As mentioned earlier, a positive pulse is required to trigger the SCR at the cathode gate.

**d) Application of low voltage at anode:**
Applying a low voltage at the anode does not trigger the SCR. The gate needs to be triggered with a positive pulse to turn on the SCR.

Therefore, the correct method of triggering an SCR is by applying a positive pulse at the cathode gate.