* Salinity: This is the most defining characteristic of ocean water. Salinity refers to the amount of dissolved salts in the water. It's a major property that influences the density, freezing point, and chemical reactions within the ocean.

Let's look at the other options:

* B) Mineral content: While ocean water does contain minerals, salinity encompasses the mineral content. It's not a distinct property separate from salinity.

* C) Pressure: Pressure is a property of the ocean due to its depth, but it's not a defining property of the water itself.

* D) Plankton content: Plankton are living organisms found in the ocean. They are part of the ocean's ecosystem, but not a fundamental property of the water itself.

Therefore, salinity, salt, or sodium are the most major property of ocean water.