Final answer:
The most likely observation of the Doppler effect that led to the discovery that the Universe is expanding is that when observed from Earth, light spectra from stars in other galaxies have redshifts.
Step-by-step explanation:
The most likely observation of the Doppler effect that led to the discovery that the Universe is expanding is B. When observed from Earth, light spectra from stars in other galaxies have redshifts.
The Doppler effect explains that the observed spectrum of light from a moving light source is different than that observed from a stationary light source. When a light source is moving away from an observer, the observed spectrum of light has lower frequencies, resulting in a redshift. This is what astronomers observed when they studied the light spectra from stars in other galaxies, indicating that the galaxies are moving away from us and that the Universe is expanding.