The common element of the​ nation's answers to those key economic questions has​ been:

Question

A.
a centralized authority assuming responsibility for addressing them.
B.
a centralized authority assuming responsibility for what and how many items to​ produce, and private parties assuming responsibility for how scarce resources will be organized in the​ items' production, and who will be able to obtain the items.
C.
the price system assuming responsibility for addressing them.
D.
a mixed economic system assuming responsibility for addressing them.

Answer 1

D. a mixed economic system assuming responsibility for addressing them.

Answer 2

Incorrect. This might help.
what and how many items to​ produce, how scarce resources are used in producing the​ items, and who can obtain the items produced.

Answer 3

B. a centralized authority assuming responsibility for what and how many items to​ produce, and private parties assuming responsibility for how scarce resources will be organized in the​ items' production, and who will be able to obtain the items.