Final answer:
The correct answer is D. period. When the period of a wave increases, its frequency must decrease due to the inverse relationship where frequency is the inverse of period (f = 1/T). Increasing amplitude impacts energy but does not directly affect a wave's frequency.
Step-by-step explanation:
When the period of a wave increases, its frequency must decrease. The period (T) is the time it takes for one complete cycle of the wave to pass a given point, and frequency (f) is the number of cycles per unit time. According to the relationship f = 1/T, we can see that frequency is the inverse of the period. Therefore, if the period increases, the frequency decreases.
The energy of a wave is related to its amplitude; specifically, the energy carried by a wave increases with its amplitude. The amplitude of a wave does not directly affect its frequency, and thus increases in amplitude do not necessitate changes in frequency. When we speak louder, the amplitude of the sound wave increases, which corresponds to a larger energy of the sound wave.
Furthermore, as the amplitude of a sound wave decreases with distance from its source, this is due to the spreading out of the wave's energy over a larger area. It is not directly because of changes in frequency or wave speed. Therefore, for the given question, the correct answer is D. period.