Answer: D) Salt water is more dense than freshwater.
Explanation: Suppose that you have a glass of water, and you put a little amount of salt in it and mix it.
Now you may see that there is more matter in the glass (the volume of the solution inside the glass also may increase, but not in an appreciable magnitude compared with the increase in mass). And you may know, the density is calculated:
Density = Mass/volume
So if you have more mass in the same volume, this means that the density increases.
And is easier to float in a denser medium than in a low density medium (is easier to float in honey than in water, for example)
So if the medium if denser, the boat will float more easily.
Then the right answer is option D: Salt water is more dense than freshwater