How can you restrict changing access to a public instance attribute within the class?

To restrict changing access to a public instance attribute within a class in ABAP, utilizing the READ-ONLY addition while keeping it in the public section is an effective method. This allows the attribute to be accessed publicly but prevents any modifications to it after its initial assignment, ensuring that it remains unchanged throughout the object's lifecycle.

Making it a private attribute restricts access to within the class itself, which effectively prevents any external changes, but it also limits visibility. Therefore, while both making it private and applying the READ-ONLY addition are valid methods of protection, they serve slightly different purposes in terms of access control.

Using both strategies together — making an attribute private and also declaring it read-only — maximizes protection against unwanted changes while controlling access. However, utilizing the READ-ONLY addition in the public section is a specific technique designed to prevent changes while still allowing external access to the value. This combination makes the chosen answer plausible and correct, as it encompasses both options that limit access while defining appropriate visibility.