Why does water sink in polar regions?

a. evaporation due to strong wind creates gyres.
b. water gets colder and saltier there, increasing its density.
c. southern oscillation currents force water down as they strike land.
d. the great ocean conveyor belt transports water around the globe over many
years' time.

Because colder water has more density due to state changing, added with saline/salt, making it even more dense, causing it to sink.

B is your answer.

you didn't give any options but heres what i can tell you: an inference is likely to change based on additional observations. the latter is something that you have seen or sensed using your sensory organs while the former is like a conclusion out from what you have observed.