The correct answer is A. 10% less.
Sea water contains about 35 grams of salt per liter, while fresh water contains about 0 grams of salt per liter. The salt in sea water makes it more dense than fresh water, which means that there are fewer molecules of oxygen in a given volume of sea water than there are in fresh water. This is because the salt ions in sea water take up space that could otherwise be occupied by oxygen molecules.
As a result, sea water contains about 10% less oxygen than fresh water. This can have a number of effects on marine life, as many organisms require oxygen to survive. For example, fish have to swim to the surface of the water to breathe, as the oxygen concentration is higher there. Additionally, some organisms, such as coral reefs, can be damaged by low levels of oxygen in the water.
It is important to note that the amount of oxygen in sea water can vary depending on a number of factors, such as temperature, salinity, and currents. For example, sea water that is warmer or more saline will contain less oxygen than sea water that is cooler or less saline. Additionally, sea water that is near the surface will contain more oxygen than sea water that is deep below the surface.