Final answer:
The most appropriate question to start the investigation would be to explore how differences in density between salt water and fresh water affect the speed of sound (option C), given that salt water is denser and sound speed is documented to be higher in denser media.
Step-by-step explanation:
The student wishes to determine the possible reasons for why sound travels at different speeds through salt water and fresh water. The most appropriate question for them to start their investigation would be: Do differences in density of salt water and fresh water affect the speed at which sound travels? This is because sound waves are pressure waves and the speed of sound will be influenced by the medium's density. Salt water has more salt, making it denser (1.022 kg/L) than fresh water (1.000 kg/L), which could affect how sound waves travel through these media. Given the density variations between fresh water and salt water and the data that sound speed is higher in salt water (1546 m/s) compared to fresh water (1509 m/s) at 30 degrees Celsius, the implication is that density has a significant role.
Moreover, since the temperatures given for both types of water are the same, we can rule out temperature as a factor under these conditions. Differences in mass are related to differences in density, but density is a more direct measure affecting sound propagation in this context.