(b) — PWM needs more power than PPM. TRUE. In pulse-width modulation the pulse length varies with the message, so long pulses consume energy for their whole duration and the average transmitted power varies with the signal. In pulse-position modulation every pulse has the same fixed width and amplitude — only its timing carries the information — so the average power is constant and lower. That is precisely why PPM is preferred where power is scarce, and PWM is preferred where simple demodulation matters (a PWM signal recovers with a low-pass filter alone).

(d) — PCM is a digital modulation technique. TRUE. PCM samples, quantises and then encodes each sample as a binary word. The final output is a stream of bits, not a continuously varying pulse parameter, so PCM is genuinely digital — unlike PAM, PWM and PPM, which are analogue pulse modulations with a discrete-time but continuous-valued parameter. Being digital brings regeneration, error correction and multiplexing, which is why PCM became the backbone of telephony.

(a) — PAM detected by a band-pass filter. FALSE. A PAM signal is recovered by passing the flat-topped samples through a low-pass filter of cut-off ≥ the message bandwidth: the filter interpolates between samples and rebuilds the original waveform, exactly as the sampling theorem prescribes. A band-pass filter would remove the baseband content that carries the message.

(c) — noise reduced by increasing the sampling rate. Not accepted as stated. Sampling faster than the Nyquist rate only adds guard band against aliasing; the quantisation noise power in PCM is set by the step size, i.e. by the number of bits:

\(\left(\dfrac{S}{N}\right)_{dB}\approx 6.02n+1.76\)

so noise is cut by adding bits, not by sampling oftener.

So the correct statements are (b) and (d), which is option 3.

Hence, the correct answer is (b) and (d).