When water is heated from 0°C to 4°C, its density increases. This is because water is unique in the sense that its density decreases as it gets colder, until it reaches its maximum density at approximately 4°C. Beyond this point, when water is heated further, its density starts to decrease again.

As water is heated, the molecules gain thermal energy and begin to move more rapidly. At temperatures below 4°C, the increased molecular motion causes the water molecules to spread out, leading to a decrease in density. However, at 4°C, the hydrogen bonds between water molecules become more organized, and this causes a dense arrangement of molecules, resulting in the maximum density.

When water is heated beyond this temperature, the additional thermal energy disrupts the hydrogen bonds. As a result, the molecules become less organized and spaced out, leading to a decrease in density. Therefore, the correct answer is option-2: the density of water increases when heated from 0°C to 4°C.