As you move to a higher elevation (e. g. in the mountains), the air pressure drops significantly. in turn,
a) the temperature of water increases and the density goes up as well.
b) the temperature of water drops and the density of it goes down as well.
eliminate
c) the evaporation rate of water increases at all temperatures causing an increase in volume.
d) the evaporation rate of water decreases at all temperatures causing a decrease in volume.