The correct answer is C) Type of bottom. The type of bottom (seafloor) has the greatest effect on the amount of gain required to obtain a fathometer reading. This is because the composition and density of the seafloor material directly impact the strength of the echo signal received by the fathometer. Harder, denser bottoms like rock or sand will reflect the sound waves more strongly, requiring less gain to obtain a readable depth measurement. Softer, less dense bottoms like mud or silt may require significantly more gain to amplify the weaker echo signal. The other factors like water temperature, atmospheric pressure, and salinity can affect sound propagation and the fathometer's performance, but they have a less direct impact on the required gain levels compared to the seafloor composition.