When does the "Display Style" icon become available on the heads-up toolbar?

The "Display Style" icon becomes available on the heads-up toolbar specifically when a drawing view is selected from the sheet. This is because the icon is tied to the properties and display modes of the drawing views themselves. When a view is selected, it allows users to modify how that particular view is displayed—be it in a wireframe, hidden lines removed, or shaded style.

In the context of the other options, the first choice suggests that the icon would be available as soon as any drawing is opened, which isn't the case since no specific view is active until one is selected. The third option mentions the adjustment of the sheet scale, but this action does not influence the display style options directly. Lastly, the fourth option refers to the activation of the Bill of Materials (BOM), which is not related to the visual display of drawing views and, therefore, does not activate the Display Style icon.

Overall, the activation of the Display Style icon aligns with the selected drawing view's requirements, making it context-dependent.