Final answer:
The answer to the question is c) Estuaries, as they are not sources of salts to the ocean but rather areas where freshwater and ocean water mix, resulting in varied salinity levels.
Step-by-step explanation:
The correct answer to the question "All of the following EXCEPT are a source of salt for the ocean" is option c) Estuaries. While runoff and hydrothermal vents are sources of salts, including sulfates, to the ocean, estuaries are not a source of salt but rather a mix of salty ocean water and freshwater from rivers, where salt levels can vary.
Estuaries typically have varying levels of salinity due to the mixing of freshwater with ocean water, not contributing additional salts to the ocean. Runoff from land can carry salts into the ocean, and hydrothermal vents release minerals and salts from the Earth's crust into the deep ocean, raising the salinity levels in those areas. Therefore, estuaries would not be considered a source of salts for the ocean compared to the other options.