What will happen if a private IP address is assigned to a public interface connected to an ISP?

What will happen if a private IP address is assigned to a public interface connected to an ISP?

A.
Addresses in a private range will be not be routed on the Internet backbone.

B.
Only the ISP router will have the capability to access the public network.

C.
The NAT process will be used to translate this address to a valid IP address.

D.
A conflict of IP addresses happens, because other public routers can use the same range.