Final answer:
A sound wave speeds up (option b) when it moves from water to air because of the difference in acoustic impedance, meaning it encounters less resistance in air despite air's lower density.
Step-by-step explanation:
When a sound wave moves from water to air, it speeds up. This occurs because sound waves travel faster in denser media, such as water, and slow down when they enter less dense media like air. However, in this case, water has a higher density, but sound travels faster in air due to its lower acoustic impedance. So when sound moves from water to air, it encounters less resistance and thus speeds up. To understand this phenomenon, it's useful to remember how waves behave: when the period of a wave increases, its frequency decreases, denoting an inverse relationship. Also, if a sound source moves towards an observer, the sound becomes more high-pitched due to the Doppler effect.