Answer:
D. The number of highly-skilled workers migrating from Adelphia to foreign countries has fallen.
Step-by-step explanation:
If the decrease in the working population of Adelphia was caused by workers migrating from Adelphia to other nations, then if the rate of emigration has fallen (probably because the local economy is improving), the total working population of Adelphia should either remain the same or increase.
A lot of people that emigrate away from their home countries do it because they are seeking better job opportunities and a better living conditions. If Adelphia's economy is growing then the living conditions in the country should improve, so less people have to emigrate the country seeking a better life. For example, if living standards had been great in Europe during the last 3 centuries, then the US would have a very small population and there would be much more Europeans (and Latin Americans, Chinese, etc.) in their home countries.