In a scenario where non-functional requirements are not specified, a model-based technique using operational profiles is the most effective method for testing reliability. An operational profile is a quantitative model of how a system will be used in the field, detailing the frequency and probability of various user inputs or operations. By creating and testing against this model, the test team can simulate real-world usage. This allows for the selection and prioritization of test cases that target the most frequently used or critical functions, providing a statistically valid measure of the system's reliability without pre-defined non-functional requirements.

B. Ambiguity reviews are a static analysis technique for finding defects in documentation; they do not select or execute tests for system characteristics.

C. Test condition analysis is a general technique that requires a defined test basis (e.g., requirements), which is explicitly missing for non-functional aspects.

D. Cause-effect graphing is a functional, black-box test design technique for testing complex logical conditions, not for assessing reliability based on usage patterns.

1. Musa, J. D. (1993). Operational profiles in software-reliability engineering. IEEE Software, 10(2), 14–32. In Section "What is an operational profile?" (p. 15), it is defined as a set of operations and their probabilities of occurrence, which is fundamental to guiding test selection for reliability engineering. The entire paper establishes this as the primary technique for the scenario described. DOI: https://doi.org/10.1109/52.199724

2. Lyu, M. R. (Ed.). (1996). Handbook of Software Reliability Engineering. McGraw-Hill. Chapter 5, "The Operational Profile," by J.D. Musa, details the process of developing and using operational profiles to guide test case selection for reliability measurement and improvement (pp. 167-170).

3. Whittaker, J. A., & Voas, J. M. (2000). Toward a more reliable theory of software reliability. IEEE Computer, 33(12), 36-42. This article discusses the importance of usage-based testing and operational profiles (p. 38) as a practical and effective means of assessing software reliability in the absence of complete formal specifications. DOI: https://doi.org/10.1109/2.889088