Answer:
B. Factors of Gross National Products
Step-by-step explanation:
I did an elimination strategy on this one. Here's my logic:
For A, GDPPC cannot be considered a "key socio-economic indicator", because everybody in a country, no matter their socioeconomic status, has the same GDPPC obviously.
For C, GDPPC isn't considered physical... I think?
For D, It's not political and it's not strictly in the US