Question

if an observer notices that more waves pass by a certain point every second, what can be concluded? its frequency has decreased. its amplitude has decreased. its wavelength has gotten longer. its frequency has increased.

Explanation:

Frequency is defined as the number of waves passing a fixed point per second. If more waves pass the point each second, this directly matches the definition of increased frequency. Amplitude relates to wave height, wavelength is wave distance, neither are tied to wave count per second.

Answer:

D. Its frequency has increased.