Which research method involves participants organizing features of a user interface?

The research method that involves participants organizing features of a user interface is card sorting. This technique is used to understand how users think about the organization and labeling of information. In a card sorting exercise, participants are given cards that represent different content elements or features of a user interface, and they are asked to group these cards in a way that makes sense to them.

Card sorting helps designers gain insight into users' mental models, revealing how they categorize and prioritize information. This can inform the structure of navigation, improve usability, and enhance the overall user experience by aligning the interface with users' expectations and natural thought processes.

The other methods mentioned, such as case studies, focus groups, and usability tests, serve different purposes within user research. While case studies provide in-depth analyses of specific instances, focus groups gather collective opinions on concepts or designs, and usability tests evaluate how easily users can navigate a specific interface. However, only card sorting directly engages participants in organizing elements, making it the correct choice for this question.