Which of the following is NOT one of the features of a system?

Efficiency is not typically considered a fundamental feature of a system, primarily because it is more of an assessment or evaluation metric rather than a defining aspect of the system itself. The core features of a system usually include aspects that describe how it operates, its structure, and its objectives.

A system's life cycle refers to the various stages it goes through from inception to retirement, indicating its longevity and evolution. Functions are the specific roles and tasks that the system is designed to perform, outlining its purpose and utility. Performance characteristics encompass various attributes that quantify how well a system meets its intended functions and requirements under specified conditions.

Efficiency, while an important metric for evaluating the effectiveness of a system in achieving its functions, does not inherently describe the structure or roles of the system itself. Therefore, it is the correct answer as it does not align with the core features that define what a system is.