Explanation:

- Atmospheric pressure is low:

- The correct answer is that at high altitudes, atmospheric pressure is low. This reduction in pressure causes the boiling point of water to decrease because less energy is required for water molecules to transition to the vapor phase.

- Atmospheric pressure is high:

- This is incorrect because high altitudes have lower atmospheric pressure, not higher.

- Temperature is low:

- While temperatures can indeed be lower at high altitudes, this is not the reason for the lower boiling point of water.

- None of these:

- This option is incorrect, as the lowering of boiling point is directly related to atmospheric pressure.