What characteristic of defensive design facilitates modification of a program?

Maintainability is a crucial characteristic of defensive design that facilitates the modification of a program. When software is developed with maintainability in mind, it is structured in a way that allows for easier updates, bug fixes, and improvements over time. This involves writing clear, modular code, adhering to coding standards, and ensuring that documentation is thorough and accessible.

Such practices make it simpler for developers—not only those who created the software but also for others who may work on it later—to understand the code and its structure. This supports efficient modifications and reduces the likelihood of introducing new errors when changes are made. In contrast to the other options, maintainability specifically addresses the ease with which a program can be modified, enhancing its longevity and adaptability to changing requirements.

Portability refers to how easily software can be transferred from one environment to another, scalability focuses on a program's ability to handle increased loads, and compatibility deals with a program's ability to operate with other systems or software. While these characteristics are important in their own contexts, they do not specifically address the modification process of a program as directly as maintainability does.