Which of the following remains unchanged when a sound wave is...

When a sound wave travels from one medium to another, its frequency remains unchanged because frequency is a fundamental characteristic of the source producing the sound.

The wave velocity changes as it depends on the properties of the medium (sound travels faster in water than in air).

Since velocity $v = f \lambda$ and frequency $f$ is constant, the wavelength $\lambda$ must change proportionally with the velocity.

The wave number $k = \dfrac{2\pi}{\lambda}$ also changes because the wavelength changes.