Question

When the source of a wave is moving relative to an observer of the wave, which of the following is true?

Answer 1

Answer:

Observers ahead of the wave observe an apparent increase in wave frequency  

Explanation:

This is the Doppler Effect. The frequency of a wave increases when the source approaches the observer and decreases as the source recedes.

B, C, and D are wrong. Observers behind the source notice a decrease in frequency.