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

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.

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

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.

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.

The correct option is D. a mixed economic system assuming responsibility for addressing them.

A mixed economic system is characterized by a combination of market forces and government intervention. In this system, the government plays a role in addressing key economic questions such as what and how many items to produce, how scarce resources will be organized in production, and who will be able to obtain the items. This means that both private parties and a centralized authority have a responsibility in answering these questions.