What is state transition testing?

State transition testing is a black-box test design technique specifically aimed at ensuring that software behaves as expected when transitioning from one state to another. This method is particularly useful in systems that have distinct states, such as applications with user interfaces, workflow systems, or state-dependent functionalities.

When using state transition testing, the tester identifies the various states of the system and the transitions that can occur between those states based on specific events or conditions. By systematically testing each possible state and its transitions, it is possible to identify defects that might arise when transitioning between states, thereby enhancing the software's reliability and robustness.

The other options do not align with the definition of state transition testing. For example, usability testing focuses on the user experience and interface quality, while security testing targets vulnerabilities in software. Additionally, a structured approach to unit testing is more concerned with testing individual components of software in isolation rather than focusing on state changes. Thus, the emphasis of state transition testing on state changes makes it the correct answer.