For an SCR with turn-on time of 5 microsecond, an ideal trigger pulse should have

The correct answer is option 3.  

Concept:

Pulse Signal:

• To reduce gate power dissipation, SCR firing circuits generate a single pulse or a train of pulses instead of a continuous DC gate signal.
• This allows precise control of the point at which the SCR is fired.
• In addition, it is easy to provide electrical isolation between the SCR and the gate trigger circuit.
• Electrical isolation by means of a pulse transformer or optical coupler is important if several SCR's are gated from the same source.
• Isolation also reduces unwanted signals such as transient noise signals that could inadvertently trigger a sensitive SCR.
• Since in a pulse transformer induced emf is proportional to the frequency, therefore a high frequency train pulse is used to reduce the core size.
• Higher frequencies allow the reducing component size.

Solution:

For an SCR with turn-on time of 5 microseconds, an ideal trigger pulse should have short rise time with pulse width = 6 μsec.

Additional Information

Gate triggering or firing circuits:

These are used to give the necessary gate pulse to trigger or turn ON the SCR.

Basically, there are two types of firing circuits used frequently.

Resistance triggering circuit:​​​

It is the simplest triggering method.

The firing angle range is 0∘ < α < 90∘ 

The variable resistance in the circuit is needed to change the firing angle.

RC triggering circuit:

The limited range of firing angles in the resistance triggering is overcome in the RC triggering circuit.

The firing angle range is  0∘ < α  < 180∘