UK Air Mass Stability MET
Air masses that are warmed from below will be increasingly unstable (less stable.)
Air masses that are cooled from below will be increasingly stable.
To answer the question, think about the temperature where the air is coming from and think about whether it warms or cools on the way towards the UK. Broadly speaking, if it is moving south it warms and becomes less stable; if it is moving north it cools and becomes more stable. This generalisation does not apply to two of the air masses, however: polar continental (where the warming/cooling is season dependent) and the returning polar maritime (where the stability will vary depending on sea track.)