Answer: Option (c) is the correct answer.
Explanation:
Density is defined as mass present in per unit volume.
Mathematically, Density = 
Since, density is directly proportional to mass. So, more is the mass that is, more is the number of molecules closer to each other more will be the density of substance.
So, when we cool water then decrease in temperature causes the molecules to slow down. As a result, they come closer to each other due to which there occurs increase in mass of the water. Hence, density of cold water increases.
Whereas when water is warm then molecules have more kinetic energy. So, they move away from each other due to more number of collisions. Hence, there will decrease in density of warm water.
Thus, we can conclude that cold water is denser than warm water.