Final answer:
In circuit analysis, F.S. is likely to mean Full Scale, denoting the maximum output of a measurement device. However, in the context of sound and waves, F.S. could relate to the Frequency of the Source or Frequency Spectrum.
Step-by-step explanation:
In the context of circuit analysis, F.S could stand for Full Scale, which refers to the maximum output reading of a measuring device such as an ammeter or voltmeter in a circuit. However, it is important to ensure the context matches this interpretation, as F.S might have other meanings in different fields. If the topic is related to sound and waves, as indicated by the additional information provided about frequency modulation and the relationship between the speeds of sound, observer, and source, F.S. could potentially represent Frequency of the Source. Nevertheless, the choices provided (A) Frequency and Signal, (B) Forward System, (C) Field Strength, (D) Frequency Spectrum do not explicitly match the mentioned contexts. Among these choices, the one that seems more related to the context of sound and waves would be Frequency Spectrum (D), which refers to the range of frequencies contained in a sound or signal, which is important in understanding frequency modulation as described in the provided reference material.