To answer this question, the user needs to know what is meant by testable software and what are its characteristics.

Testable software is software that can be easily tested to ensure that it meets the requirements and specifications provided. Characteristics of testable software include:

A. Simplicity: Testable software should be easy to understand and use. When software is simple, it is easier to test and identify issues.

B. Stability: Testable software should be stable and reliable. This means that it should perform consistently and not produce unexpected results.

C. Observability: Testable software should be observable, meaning that its behavior can be easily monitored and studied. This allows for easier identification of issues and faster resolution of problems.

Based on the above explanation, the correct answer is:

The Answer is: D. All of the above