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**Hint:**To answer this problem, we can use the relationship between wave speed, frequency and wavelength. It gives us variation in speed when wavelength is varied. Then, you can also use the relation between density of a medium and the speed of sound to answer this question. With help of these relations, you can find which quantity remains unchanged when sound travels from air to water. These relations will hold true even if the sound is travelling from water to air.

**Formula used:**

$\nu = \dfrac {c}{\lambda}$

**Complete answer:**

Water is denser as compared to air. So, the sound waves travel faster in water as compared to air. So, when sound travels from air to water its speed changes.

When sound travels from one medium to another, some part of it gets reflected while only some part gets transmitted. The intensity of transmitted sound will be lesser as compared to the original sound. Thus, the intensity also changes.

Frequency is defined as the number of cycles per second. These numbers of cycles depend on the source producing sound. When it travels from air to water, its source remains the same and the length of a second does not change, So, the frequency remains constant.

We know, relation between frequency and wavelength is given by.

$\nu = \dfrac {c}{\lambda}$

$\Rightarrow c = \nu \lambda$

Where, $\nu$ is the frequency which is constant

c is the speed of wave

$\lambda$ is the wavelength

From the above equation, we can infer that the speed is proportional to wavelength. So, if speed changes, wavelength changes too.

Hence, when sound travels from air to water, the quantity that remains unchanged is frequency.

**So, the correct answer is “Option D”.**

**Note:**

Students should remember that sound waves need a medium to travel, it cannot travel through vacuum. The speed of sound increases with the increase in closeness of the particles. The speed of sound is greater in solids as compared to liquids and the speed of sound is greater in liquid when compared with gases. This is because the particles are much closer in solids as compared to liquids and gases.

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