Introduction:
An SCR (Silicon Controlled Rectifier) is a semiconductor device that can control the flow of electric current. It is widely used in various applications such as power supplies, motor drives, and lighting control. However, one of the limitations of an SCR is its sensitivity to high di/dt (rate of change of current) conditions. High di/dt can cause false triggering and damage the SCR. To protect the SCR against high di/dt, certain measures need to be taken.

Options:
Let's analyze each option to understand how it helps in protecting the SCR against high di/dt:

a) A capacitance in series with SCR:
Connecting a capacitance in series with the SCR will not provide any protection against high di/dt. Capacitors allow AC signals to pass through them but block DC signals. In this case, high di/dt is an AC signal, so the capacitance will not have any effect on it.

b) A capacitance in parallel with SCR:
Connecting a capacitance in parallel with the SCR can provide some protection against high di/dt. Capacitors can store and release electrical energy, acting as a low-pass filter. They can absorb high-frequency components of the di/dt and reduce its amplitude before it reaches the SCR. However, this method is not as effective as other options.

c) An inductance in parallel with SCR:
Connecting an inductance in parallel with the SCR will not provide any protection against high di/dt. Inductors resist changes in current, so they would not allow high di/dt to pass through. However, this would also prevent the SCR from functioning properly.

d) An inductance in series with SCR:
Connecting an inductance in series with the SCR is the most effective method to protect it against high di/dt. Inductors resist changes in current, so they can limit the rate at which the current rises. By adding an inductance in series with the SCR, the di/dt can be controlled and reduced to a safe level, preventing false triggering and damage to the SCR.

Conclusion:
To protect an SCR against high di/dt, the most effective method is to connect an inductance in series with the SCR. This helps to limit the rate at which the current rises and prevents false triggering and damage to the SCR.