The correct answer is option 3, which states that the density of water is maximum at 4°C.

To understand why this is the correct answer, we need to consider the properties of water. Normally, as substances cool down, their density increases because the molecules become more closely packed together. However, water behaves differently due to its unique molecular structure.

As water cools from room temperature (around 20°C), its density generally increases until it reaches 4°C. At this point, water molecules form an open, hexagonal lattice structure due to the orientation of hydrogen bonds. This lattice structure causes the molecules to space out slightly more than they do at higher temperatures, resulting in a decrease in density.

Further cooling below 4°C causes the water molecules to arrange more closely together, which would typically result in an increase in density. However, the expansion of water as it freezes into ice counteracts this increase in density. Ice is less dense than liquid water, which is why it floats.

Therefore, the density of water is indeed maximum at 4°C. This is an important property in the natural environment, as it allows bodies of water to freeze from the top, insulating the organisms living underneath and providing a stable environment for aquatic life.