Mary's requirement that she should be able to change the screen color whenever she wants represents which of the following Quality Requirements?

Flexibility is a quality requirement that refers to the ease with which a system can be modified to be used in different ways or to adapt to different user preferences or environments. Mary's requirement to change the screen color on demand is a classic example of flexibility. The system is designed to be adaptable to her specific visual preference without requiring any underlying code changes, thus providing a flexible user experience. This capability allows the system to cater to a wider range of user needs and contexts.

A. Reusability: This refers to the ability of a software component to be used in multiple different systems. The requirement is about a feature within a single system, not reusing parts of it elsewhere.

C. Portability: This concerns the ease of transferring the software from one hardware or software environment to another (e.g., from Windows to Linux). It is unrelated to user interface customization.

D. Maintainability: This relates to the ease with which developers can correct faults, improve, or adapt the software. The requirement is from a user's perspective, not a developer's maintenance perspective.

1. Sommerville, I. (2016). Software Engineering (10th ed.). Pearson. In Chapter 4, "Requirements Engineering," non-functional requirements are discussed. Flexibility is described as the system's ability to be adapted to different business or user needs. User-configurable settings are a direct implementation of this principle (pp. 108-110).

2. Pohl, K. (2010). Requirements Engineering: Fundamentals, Principles, and Techniques. Springer. Chapter 2, Section 2.2.2, "Quality Requirements," defines various quality attributes. Flexibility is presented as a system's capability to adapt to changes in its environment or user needs, which directly supports the scenario of a user changing UI settings (pp. 38-40).

3. University of Toronto, Department of Computer Science. (n.d.). CSC407/2104 - Software Architecture, Lecture 5: Quality Attributes. Course materials often define flexibility as the ability of a system to be changed or customized to suit different users or tasks. Mary's requirement is a form of customization, hence flexibility.