The air begins to rise along Path a.  Rising air expands and cools.  As the air cools, the relative humidity will increase, but the mixing ratio remains constant along Path a. 

Path a ends and Path b begins when the RH becomes 100%, the rising air has cooled to its dew point.  The air continues to rise and cool along Path b.  Water vapor begins to condense and fall to the ground as rain.  Because the air is losing moisture, the mixing ratio will decrease.  The RH remains constant at 100%.

Path c begins when the air begins to flow downhill.  Sinking air is compressed and warms.  The RH will drop below 100% and the cloud disappears.  No moisture is added to or removed from the air along Path c.  The mixing ratio will remain constand and the relative humidity is decreasing as the air warms along Path c.