What shows what functions are available in a design?

The correct choice indicates "Visibility" as it relates directly to the principle of design where users can easily see what functions are available to them within an interface. When the functions are visible, it enhances user interaction by providing clear cues about what actions can be taken. Good visibility allows users to understand what elements are interactive, guiding them towards making informed decisions as they navigate the design.

In the context of design, visibility is crucial because it minimizes confusion and reduces the learning curve associated with using a particular interface. Elements should be designed in a way that makes their functionality clear and straightforward, so users do not have to guess what they can or cannot do.

Other concepts, while essential in design, focus on different aspects. Signifiers indicate what is possible but do not necessarily ensure that users can readily see those functionalities. Conceptual models pertain to the mental representation that users form about how something works, which doesn't directly involve visible cues. Affordances suggest how an object can be used but rely on users recognizing these cues; hence, without visibility, affordances might not effectively communicate their functionalities.