'Memorability' refers to what aspect of user interaction with a design?

Memorability specifically pertains to a user's ability to recall how to use a design effectively after not engaging with it for a period of time. It focuses on the retention of knowledge and skills related to the design, which is particularly important for applications or systems that users may not interact with daily. A design that is memorable allows users to easily return to the interface and perform tasks without needing to relearn how to use it, thereby enhancing overall user satisfaction and efficiency.

In contrast, the other options touch on different aspects of user interaction. Speed of task completion emphasizes how quickly a user can perform specific actions within the design, which is more about efficiency than memorability. Completeness in achieving specific goals relates to whether users can accomplish their intended tasks, which is essential but does not inherently reflect their ability to remember how to use the system later. The ease of learning the system focuses on how intuitive the design is for new users to grasp initially, rather than the retention of that knowledge over time. Thus, understanding memorability is crucial for designing user interfaces that users can come back to without significant effort in re-familiarization.