A snubber circuit is used to protect a semiconductor device, such as an SCR (Silicon Controlled Rectifier), from voltage spikes or transients. It consists of an RC series circuit connected in parallel with the semiconductor device. The assertion (A) states that a snubber circuit has an RC series circuit in parallel with an SCR, which is correct.

The reason (R) given is that the voltage across a capacitor cannot change instantaneously. This is also correct and provides a valid explanation for why an RC circuit is used in a snubber circuit.

Let's discuss each point in detail:

1. Snubber Circuit:
- A snubber circuit is used to protect semiconductor devices from voltage transients that can damage them.
- It is commonly used with devices like SCRs, which are sensitive to rapid changes in voltage.
- The primary purpose of a snubber circuit is to limit the rate of change of voltage and prevent voltage spikes or transients from occurring.

2. RC Series Circuit:
- An RC series circuit consists of a resistor (R) and a capacitor (C) connected in series.
- The resistor limits the current flow and the capacitor stores electrical charge.
- When a voltage transient occurs, the capacitor charges or discharges through the resistor, which helps to smooth out the voltage waveform.

3. Parallel Connection with SCR:
- The RC series circuit is connected in parallel with the SCR in a snubber circuit.
- When a voltage spike or transient occurs, the series RC circuit provides a path for the transient current to flow.
- The resistor limits the rate of change of current, and the capacitor absorbs or releases the excess charge, preventing it from reaching the SCR.

4. Voltage across a Capacitor:
- The reason (R) states that the voltage across a capacitor cannot change instantaneously.
- This is due to the property of a capacitor to store electrical charge and resist changes in voltage.
- When a voltage transient occurs, the capacitor initially resists the change in voltage and begins charging or discharging through the resistor.
- This charging or discharging process takes time and prevents the voltage across the capacitor from changing instantly.

In conclusion, both the assertion (A) and the reason (R) are correct. The reason provides a valid explanation for why an RC series circuit is used in a snubber circuit. The resistor and capacitor in the RC circuit help to limit the rate of change of voltage and protect the semiconductor device from voltage transients.