Final answer:
Robustness is not a characteristic of testability. Testability is associated with properties like operability, observability, and simplicity, which facilitate the testing process. Robustness pertains to a system's capability to handle stress and does not directly contribute to ease of testing.
Step-by-step explanation:
The characteristic that is not a part of testability is Robustness. Testability refers to the ease with which a system or components can be tested. It is crucial in the design of systems for which verification is important. Let's look at the options provided:
Therefore, the correct answer is D. Robustness.