What action must be taken to cancel opening a chart dialog in PowerPoint?

To effectively cancel opening a chart dialog in PowerPoint, closing the dialog using the close button is the appropriate action. This action allows you to exit the dialog box without making any changes or selections. When the close button is clicked, it will terminate the chart dialog and return you to the previous interface, enabling you to continue working on your presentation without disruption.

The other methods suggested are less effective or impractical. For instance, clicking the 'insert chart' button again may not necessarily cancel the dialog; it could result in a new dialog opening instead. Typing "cancel" in an input box is not a standard command for PowerPoint and would not have any effect on closing the dialog. Finally, clicking back to the last slide does not directly cancel the dialog and might lead to confusion or unintended navigation away from the current task. Thus, utilizing the close button is the most straightforward and effective method for this situation.