Which would describe a situation in which the doppler effect is occurring?

a. sheila notices an increase in volume as an object approaches her.
b. brad hears an object increase in loudness as it is vibrated at its natural frequency.
c. judy notices an increase in intensity as an object is vibrated at its natural frequency.
d. damian hears a change in pitch as an object moves past him.

The Doppler effect describes the change in frequency, or pitch, of a source that is moving with respect to the observer. The frequency of the incident wavelength is higher than the frequency being emitted when the source is travelling towards the observer, identical to the emitted frequency when the source is right next to the observer and less than the emitted frequency when the source is leaving the observer.
Therefore, the answer is D.