The study guide identifies A. Visibility of system status as the correct answer. The visual indicators communicate important information about the current state and requirements of the interface before the user attempts submission. By clearly showing which fields are required, the system makes its expectations visible rather than forcing users to discover them only after making an error.
Option B, User Control and Freedom, concerns providing users with mechanisms to recover from or reverse unwanted actions, such as canceling, backing out, or undoing operations. That is not the main issue here.
Option C, Error diagnosis and recovery, relates to helping users understand and recover from errors once they occur. Required-field indicators can help prevent such errors, but in this study-guide question the emphasis is on making necessary status and requirements visibly apparent.
Consistent patterns are especially important. If required fields are indicated differently on different screens, users must repeatedly relearn the interface. A shared pattern creates predictable behavior and enables faster form completion.
Therefore, the visual indicator acts as continuous system communication: users can immediately perceive what the form expects from them before attempting to complete the process.
Study Guide reference/topics: Usability Heuristics, visibility of system status, required fields, form design, UX Fundamentals.
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