The correct answer is D. all of the above.
Sea water is denser than pure water because it contains dissolved salts, suspended matter, and dissolved air. The dissolved salts increase the mass of the water, while the suspended matter and dissolved air increase the volume of the water. This results in a higher specific weight for sea water than for pure water.
Dissolved salts are the main reason for the higher specific weight of sea water. The average salinity of sea water is about 35 parts per thousand (ppt). This means that for every 1,000 grams of sea water, there are 35 grams of dissolved salts. The most abundant dissolved salt in sea water is sodium chloride (NaCl), which is common table salt. Other dissolved salts include magnesium chloride (MgCl2), calcium chloride (CaCl2), and potassium chloride (KCl).
Suspended matter is another factor that contributes to the higher specific weight of sea water. Suspended matter can include anything from small particles of sand and clay to larger pieces of organic matter. The amount of suspended matter in sea water can vary depending on the location and the time of year. For example, rivers and streams can carry suspended matter into the ocean, and storms can stir up sediment from the seafloor.
Dissolved air is a third factor that contributes to the higher specific weight of sea water. Air is always present in the atmosphere, and some of this air dissolves in sea water. The amount of dissolved air in sea water can vary depending on the temperature and pressure of the water. For example, warm water can hold more dissolved air than cold water, and high-pressure water can hold more dissolved air than low-pressure water.
The combined effects of dissolved salts, suspended matter, and dissolved air result in a higher specific weight for sea water than for pure water. This means that sea water is denser than pure water, and it will sink in pure water.