Which of the following scenarios will decrease the boiling point of water?

Water boils when its vapour pressure equals the pressure of the air above it. Lower air pressure means a lower boiling point.

On a mountain the air is thinner, so the atmospheric pressure is lower. Water boils at less than 100°C there.

Increasing pressure raises the boiling point. Adding salt also raises it slightly. Sea level has normal pressure, so water boils at 100°C.