Meteorological question about stability

[NecPluribusImpar]

Hello,

I'm doing my ATPL training, and already finished half of the exams for the authorities. As I was studying meteo, I encountered this question:
Question: The stability in a layer increases by advection of

1. moist air in the lower part
2. warm air in the lower part
3. cold air in the lower part
4. dry air in the upper part

As the answer in my book is very doubtful to me, I was wondering if somebody else could answer this question. Can you also give a small explication how you come to your answer.

Thanks in advance