Final answer:
Candice is editing a 'corner element' for her 2D inventory system, a term which informally refers to any user interface element positioned in a corner of the screen.
Step-by-step explanation:
Candice is working on an element located in the bottom right corner of her screen for a 2D inventory system. Considering the typical layout of graphical user interfaces, the most appropriate term for this kind of element is likely a corner element, even though this is not an official designation as the others listed like header or footer. Headers typically run across the top of a screen, footers across the bottom, and sidebars usually refer to vertical elements on either side of the main content area. A corner element, while not a standard term, could be understood informally to refer to any UI element specifically positioned in a corner of the screen, like Candice's inventory system.