On a day when there is no wind, you are moving toward a stationary ambulance sounding its siren. Compared to what you would hear if you were not moving, the sound that you hear has:

Doppler Effect

Explanation:

Doppler Effect is a term in Physics that describes the phenomenon experienced when there is change in wave frequency during the relative motion between a wave source and its observer.

In other words, it is referred to as increase or decrease in the level of sound, or other waves as the distance between source and observer is closer or farther from each other, such that it causes change in pitch, which is noticeable by the observer.

For example, when a sound object moves towards an individual or observer, the frequency of the sound waves increases, leading to a higher pitch, and at the same time, when the sound object moves away from an individual or observer, the frequency of the sound waves decreases, leading to a lower pitch. E.g passing siren from ambulance.

Hence, in this case, the right answer is DOPPLER EFFECT