Final answer:
The frequency of a sound wave remains the same when it travels from air into water, so the frequency is still 250.0 Hz.
Step-by-step explanation:
The frequency of a sound wave doesn't change when it enters a different medium. So, when a stereo speaker produces a pure tone with a frequency of 250.0 Hz in the air and then the sound wave enters some water, the frequency remains the same at 250.0 Hz. What does change, however, is the speed of the sound and its wavelength in the different medium, due to the differing densities and elastic properties of the media. Therefore, the correct answer to the question 'What is the frequency of this sound wave after it enters some water?' is (a) 250.0 Hz.