A nice simple question where the answer is definately A.
Air can hold a certain amount of water depending on it's temperature. Warmer air can hold more water vapour than colder air.
Thus, by cooling the air, it can hold less water vapour and the relative humidity increases. How does air cool? - usually by increasing in height (but not always!)
So to get an unsaturated parcel of air with say 40% RH to 100% RH (fully saturated - a cloud), then you must cool the parcel by lifting it to a higher level.
Pretty much like everyone else already said!