Which of these events happens first in earth’s polar regions?
a. the dense water sinks.
c. water molecules move closer together.
b. the water travels as a deep current.
d. cold air chills water molecules.

The correct answer is - d. Cold air chills water molecules.

When the water from the lower latitudes reaches the polar regions, the cold air starts to cool off the water molecules. As the water molecules are getting colder, they are moving closer to each other. Because they are moving closer to each other, the density of the water is becoming bigger, so it starts to sink deeper. As it is getting deeper, the water is starting to move towards the lower latitudes as a deep current.

a is the answer i believe.