Each menu item should have an . . . . . . . . that is unique within its menu.

The correct answer is: Access key.

An access key is a keyboard shortcut that allows users to access a menu item without using the mouse. It is a single character or a combination of characters that is preceded by the Alt key. For example, the access key for the File menu is F, so users can open the File menu by pressing Alt+F.

Access keys are important because they make it easier for users to access menu items, especially if they are using a keyboard instead of a mouse. They are also important for users with disabilities, such as those who have limited mobility or who are blind.

The other options are incorrect because:

• A property is a characteristic of an object. For example, the properties of a menu item include its text, its image, and its access key.
• A caption is a label that describes an object. For example, the caption of a menu item is its text.
• Lists are a collection of items. For example, a menu is a list of menu items.