The SI unit used for frequency in Doppler-shift measurements is the hertz, symbol Hz. One hertz represents one periodic event or cycle per second, so it is the natural unit for describing changes in wave frequency.
Doppler shift compares an emitted frequency with the frequency received by an observer. For sound, frequency is closely related to perceived pitch; for electromagnetic waves, frequency helps determine properties such as radio-channel position or the color and spectral behavior of light.
The unit honors German physicist Heinrich Rudolf Hertz, who provided conclusive experimental evidence for electromagnetic waves. The name hertz was adopted internationally in the twentieth century and became the standard replacement for the older phrase “cycles per second.”
The distractors refer to different quantities: the tesla measures magnetic flux density, the pascal measures pressure, and the newton measures force. A related caution is that angular frequency is often expressed in radians per second, not hertz, although the two are connected by a factor of 2π.