Simulate Step down chopper circuit using any Electrical Engineering software
The modulation index refers to that sawtooth waveform you have as an input, which is meant to be compared with a 741 (no comment), giving you a PWM output. Thus, your source is not an FM modulated sine, but a sawtooth, with PULSE(). I'll let you calculate your values, but I'll hint about the rise time that can be equal to the period, while fall time and period can be zero.

I can't quite tell if the source needs to be repeating, or not, but even if it's not, PULSE() or PWL() work just fine.

Since the request behind the scenes is to see if you're able to give proper waveforms, then you can choose some models from the internal library (2N5550 is as good as any). Don't forget to set the voltage to the value in the picture.

As a side comment, you can also eliminate R1, R2, and the two voltage sources connected to them, and keep only one source with the proper level that needs to be either .stepped, or dynamically adjusted (PULSE(), or PWL(), or...).