Hey mate,
I've not done meteorology but this is my opinion......
Saturation is the same as condensation where a vapour turns to liquid. Water vapour rises in the air and then condenses as rain clouds thus becoming saturated.
So as you go up in altitude the pressure increases and temperature drops so that rules out c) and d) because they imply the temperature stay constant. That leaves the other two and since they are only concerned with altitude it must be a) as discussed previously.
Hope this helps...and is correct.
T