Which testing type focuses on how an application behaves during real-time interactions with users?

The focus of Interactive Application Security Testing is on examining how the application behaves during real-time interactions with users. This approach assesses the security of the application while it is being actively used, simulating the environment and conditions under which actual users operate. By doing so, it identifies vulnerabilities that may not be apparent through other testing methods. Interactive Application Security Testing allows for a dynamic analysis of the application, enabling testers to observe how security flaws can be exploited in real-time scenarios, thereby ensuring a secure user experience.

In contrast, unit testing concentrates on individual components of the application without considering user interactions, while functional testing evaluates the application against defined specifications and use cases but does not necessarily capture real-time user behavior. Performance testing focuses specifically on the responsiveness and stability of the application under various load conditions, rather than on security weaknesses during user interactions. This clarifies why Interactive Application Security Testing is the most appropriate choice for the given question.