Correct Answer: Its frequency remains constant, speed increases, and wavelength increases

Step 1: Frequency Behavior Across Boundaries
When a wave travels from one medium to another (in this case, from air into water), the frequency of the wave depends solely on the source producing it. Therefore, frequency (f) remains constant when crossing the boundary between two media.

Step 2: Speed of Sound in Different Media
Sound is a mechanical wave that requires a medium to travel. The speed of sound depends on the bulk modulus (rigidity/compressibility) and density of the medium. Water is much less compressible (denser and more rigid) than air, which allows mechanical disturbances to propagate faster.
Hence, the speed of sound (v) increases when moving from air into water.

Step 3: Relationship Between Speed, Frequency, and Wavelength
The speed of a wave is related to its frequency and wavelength by the wave equation:

$v=f\times \lambda$

Since frequency ($f$) is constant, speed ($v$) is directly proportional to wavelength ($\lambda$):

$\lambda =\frac{v}{f}$

Since the speed ($v$) increases as the sound wave enters water, the wavelength ($\lambda$) must also increase to maintain the constant frequency.

Conclusion:
As the sound wave crosses from air into water, its frequency remains constant, its speed increases, and its wavelength increases.