I saw it like this Childhood is a separate stage of life (SSOL) that can only be understood in its own terms (UOT).
SSOL ==> UOT
not UOT ==> not SSOL
It then says how society treats old age as a separate stage of life and concludes that it should have its own field of psychology, or understood in its own terms. That's what answer choice (D) does. Or at least how I ended up choosing (D).
AnitaMay 16, 2018
The crux of this argument relies on: "But old age is ...fundamentally different... [as proven by] the organization of modern social & economic life." The speaker doesn't tell us why this is how we can tell they're fundamentally different, so a principle that supported that would help. It tells us that that childhood is understood as a separate state of life and treated differently. Therefore, old age should be seen as a separate stage of life and treated differently.
D does that. It tells us that if two distinct times of life are treated as fundamentally different, they can only be understood in their own distinct psychology.
