The correct answer is option 1: water expands when it freezes. This phenomenon, known as thermal expansion, occurs because water molecules form a regular lattice structure when frozen, which takes up more space than the liquid state. As a result, when water freezes, it expands and exerts pressure on the inside of the metal pipes, causing them to burst.

Option 2, which states that metal contracts more than water, is not the main reason for pipes bursting. While it is true that metal contracts at lower temperatures, the expansion of water when it freezes is the primary cause of pipe bursts during winter.

Option 3, suggesting that the outside of the pipe contracts more than the inside, is not accurate. The expansion of water inside the pipes is the primary reason for bursting, not the contraction of the pipe itself.

Option 4, stating that metal expands more than water, is also incorrect. Although metal does undergo thermal expansion, it is the expansion of water upon freezing that leads to pipe bursts, not the expansion of the metal pipes.

In summary, the main reason metal pipes burst in the winter is that water expands when it freezes, exerting pressure on the inside of the pipes.