Final answer:
Sound wave velocity increases from air to liquid due to the bulk modulus, or rigidity, of the medium, which is less compressible than air, allowing sound to travel faster. The correct answer is 4) Bulk modulus of the medium.
Step-by-step explanation:
The velocity of a sound wave increases when it travels from air to a liquid medium at the same temperature primarily due to two properties: bulk modulus (rigidity) and density of the medium. The bulk modulus is a measure of the medium's resistance to compression.
A high bulk modulus indicates a less compressible and more rigid medium, which allows sound to travel faster. However, while density usually has a slowing effect on sound speed, the greater rigidity of liquids and solids (compared to gases like air) tends to have the dominant effect, resulting in increased sound speed in these media.
The speed of sound also depends slightly on temperature because it affects the medium's density, but this is more significant in gases than in liquids and solids.
Thus, when considering a constant temperature, the option which explains why sound wave velocity increases as it travels from air to a liquid is option 4) Bulk modulus of the medium.