At 4°C, the volume of a fixed mass of water is at its minimum. This is because water is densest at 4°C due to the unique arrangement of its hydrogen bonds. As the temperature of water increases above 4°C or decreases below 4°C, the hydrogen bonds break and the water molecules move further apart, causing an increase in volume. Therefore, at 4°C, the water molecules are arranged most closely together and occupy the smallest volume.