The usability issue described directly violates the "User control and freedom" heuristic. This principle emphasizes that users need a clearly marked "emergency exit" to leave an unwanted state without a complex process. In this scenario, initiating the checkout is an action that the user might want to undo to correct mistakes. The system fails to provide a simple, safe way to go back, trapping the user in the checkout flow. The unreliable browser "back" button, which can cause data loss, is not an acceptable "emergency exit," highlighting a severe lack of user control.

B. The problem is functional, concerning the user's inability to navigate freely, not an issue with visual clutter or minimalist aesthetics.

C. The core issue is a flawed process design, not a lack of instructions or help documentation to guide the user.

D. The primary flaw is the lack of control to reverse an action, not a failure to inform the user about their current status.

1. Nielsen, J. (1994). 10 Usability Heuristics for User Interface Design. Nielsen Norman Group. Retrieved from https://www.nngroup.com/articles/ten-usability-heuristics/. (See Heuristic #3: "User control and freedom").

2. Hartson, R., & Pyla, P. S. (2012). The UX Book: Process and Guidelines for Ensuring a Quality User Experience. Morgan Kaufmann. (Chapter 11, Section 11.3.2, "Usability Heuristics," discusses Nielsen's heuristic of user control and freedom, emphasizing the need for undo/redo and easy ways to exit unwanted states).

3. Stanford University. (Course CS 147: Introduction to Human-Computer Interaction). Lecture 05: Heuristic Evaluation. The course materials explain Heuristic #3, "User Control & Freedom," as providing users with an "emergency exit" and the ability to "undo" and "redo" actions, which is precisely the feature missing in the described scenario.