Correct answer: Option A) Sea water is more dense than fresh water.
[1 Mark ]
It is easier to swim in sea water than in river water because sea water has more density than river water. Sea water is denser because salt adds to the mass of the solution and makes the salt solution denser than it would be without salt.
The denser the fluid, more the upthrust and this is why it is easier to swim in sea water than fresh river water.