What type of accessibility feature is mandatory for keyboard support?

The correct choice emphasizes the importance of a visible keyboard focus indicator as a fundamental accessibility feature for keyboard navigation. This feature is crucial because it provides visual feedback to users, indicating which element currently has keyboard focus. This is particularly important for individuals who rely solely on keyboard navigation, such as those with mobility impairments or visual impairments, as it enables them to understand their location within a webpage or application.

A visible keyboard focus indicator typically manifests as a colored outline, highlight, or other visual cues that clearly show which control is active. This facilitates navigation by allowing users to see where they are within interactive elements, ultimately enhancing the usability and accessibility of digital content.

Other options, such as invisible controls, complex character layouts, and voice command activation, do not directly address the necessity for clear visual indication of keyboard focus during navigation. Each may serve different purposes in accessibility but is not inherently tied to the keyboard support feature that provides crucial guidance for users operating through keyboard input.