The correct answer is A. Multiple Document Interface (MDI).
An MDI is a user interface that allows multiple documents to be opened and displayed within a single parent window. The parent window is responsible for managing the child windows, such as arranging them on the screen and providing access to their menus and toolbars.
The main property of an MDI is the MDI child, which is a child window that is created and displayed within the parent window. All the forms are open within MDI.
The other options are incorrect.
B. Single Document Interface (SDI) is a user interface that allows only one document to be opened and displayed at a time.
C. Single Data Interface (SDI) is a user interface that allows only one data source to be accessed at a time.
D. System Document Interface (SDI) is a user interface that allows only one system document to be accessed at a time.