What causes ocean water to become denser?
a. the salinity of water and high temperatures.
b. the salinity of water and low temperatures.
c. the concentration of oxygen and high temperatures.
d. the concentration of oxygen and low temperatures.

There are two things that cause ocean water to become denser, and those are b) the salinity of water and low temperatures.
The more salt there is, the denser the water will be - take the lake Dead Sea in Israel, Jordan, and Palestine for example.
And the lower the temperature, again, the denser the water will be - the deeper you go into an ocean, it will be more difficult to move around due to density and other things.