When two ice blocks are pressed together, they join to form one block because the increase in pressure causes the melting point of ice to lower. This means that under higher pressure, ice can melt at a lower temperature than its normal melting point of 0 degrees Celsius.

Option 1 is correct because it accurately describes the effect of increased pressure on the melting point of ice. When pressure is applied to ice, the molecules are compressed, which leads to a decrease in the melting point. This allows the ice to melt and then refreeze when the pressure is removed, resulting in the two blocks joining together.

Option 2 is incorrect because it suggests that the melting point of ice increases with pressure, which is not true. Option 3 is also incorrect as it states that the melting point remains unchanged with pressure, which is not the case. Option 4 is a general statement about the melting point of ice at standard conditions, but does not address the specific relationship between pressure and melting point.

In conclusion, increased pressure causes the melting point of ice to decrease, allowing two ice blocks to join together when pressed.