Final answer:
According to the Doppler Effect, the faster moving sound source will produce a higher-pitched sound as observed by someone it is moving towards. Therefore, the correct answer is a) The faster one.
Step-by-step explanation:
The phenomenon in question is known as the Doppler Effect, which describes the change in frequency or wavelength of a wave in relation to an observer moving relative to the source of the wave.
In the context of sound, when two sources of sound producing the same frequency are moving towards an observer at different speeds, the sound from the faster source will be perceived as more high-pitched compared to the sound from the slower source.
This happens because the faster the source of the sound is moving towards the observer, the more the sound waves are compressed, resulting in a higher frequency and a higher pitch perceived by the observer.