The correct answer is option 4 - 4°C.

Water is unique in that its density varies with temperature. Typically, as a substance gets colder, it becomes denser. However, this is only true for water until it reaches a certain point. Water is densest at around 4°C.

At temperatures below 4°C, the water molecules start to arrange themselves in a way that makes the water less dense. This is due to the formation of hydrogen bonds between the water molecules, which creates a more open structure. As a result, water expands and becomes less dense as it freezes into ice.

On the other hand, as water warms above 4°C, the average kinetic energy of the molecules increases, causing them to move more vigorously and increase the space between molecules. This leads to a decrease in density.

Therefore, at 4°C, water reaches its maximum density. As the temperature deviates from this point in either direction, the density decreases. This unique property of water is essential for the survival of aquatic animals living in cold water, as it allows them to take advantage of the increased density to support their buoyancy and movement.