Including security testing in a test approach is valid when there are concerns about unauthorized access or activities, such as the threat of unauthorized copying of applications or data (option A). This type of testing aims to uncover vulnerabilities that could be exploited to compromise the confidentiality, integrity, or availability of the system. The other options listed-availability measurements (option B), system's peak load handling (option C), and frequent software changes (option D)-relate to different aspects of testing, such as reliability, performance, and maintainability, which are not directly associated with security testing.