Which class of IPv4 addresses cannot be routed over the public internet?

The unique local addresses specifically associated with Class B are designed for use within a private network and are not routable on the public internet. They serve a similar purpose to the private address ranges outlined in the Class A, B, and C specifications but are distinct due to their defined scope and intended usage. Unique local addresses are reserved for local use and are meant to facilitate communication within an organization without conflict with public IP addresses.

In contrast, Class A, B, and C addresses can be routed over the internet when assigned as public addresses. These classes are designed for global addressing and are integral to the structure of the internet. Class E, on the other hand, is reserved for experimental purposes and is not used in standard networking; however, it is not classified as private or non-routable in the same way that unique local addresses are.

Thus, unique local addresses (often aligned with RFC 1918 private addressing) are specifically designed to ensure that they do not appear on the public internet, making them the correct choice in this context.