Which type of water is the most dense?
a. cold water with low salt content
b. cold water with high salt content
c. warm water with no salt
d. warm, salty water

The correct answer is B. cold water with high salt content
Let's see why:
- First of all, cold water is more dense than warm water. In fact, in warm water molecules move faster, so there is (on average) a larger separation between them, therefore the liquid is less dense when it is warmer
- Secondly, water with high salt content is more dense than water with no salt: in fact, salt is more dense then water, therefore if we add salt in the water, this water will be for sure more dense than pure water.
Putting togethere these two pieces of information, we can state that the correct answer is B. cold water with high salt content.