PPM and PWM are pulse analogue schemes — their outputs are not digital — option 3.

The dividing line is quantisation, not pulse shape. All four systems transmit pulses of constant amplitude, so all four look digital on an oscilloscope. What decides the classification is whether the modulating information is restricted to a finite set of values:

Why PPM and PWM are analogue. In PWM the width varies smoothly and can take any value between limits; in PPM the leading edge shifts smoothly in time. Both are sampled in time but not quantised in value — and sampling alone does not make a signal digital. A receiver must measure the width or the delay with an analogue accuracy, so noise that shifts an edge translates directly into an error in the recovered sample. There is no threshold at which noise can be cleaned away.

Why PCM and delta modulation are digital. Each sample is rounded to one of a finite set of levels and sent as a code word — eight bits per sample in PCM, a single bit in delta modulation, which transmits only whether the signal has risen or fallen since the last sample. Because the receiver need only decide between two levels, regenerative repeaters can restore the waveform exactly, and noise does not accumulate along a long link. That immunity is the whole reason digital transmission displaced analogue.

PAM completes the picture as the third pulse-analogue member: amplitude carries the information there, which makes it the most noise-sensitive of all, since noise adds directly to amplitude. PWM and PPM at least keep amplitude constant, so a limiter can strip amplitude noise before detection.

Hence, the outputs that are not digital are (i) and (iii).