Which data structure is commonly used to implement a stack or undo functionality?

• A. Stack
• B. Queue
• C. Graph
• D. Linked List

Answer: (A)

Undo functionality relies on the Last-In, First-Out principle. A stack embodies that exactly: you push each action as it happens, and when you undo, you pop the most recent action to reverse it. This arrangement makes undo simple and efficient, because the most recent change is always the one you remove first, preserving the correct order of edits. A stack is usually implemented with an array or a linked list, giving fast push and pop operations. A queue wouldn’t work for undo since it processes in first-in, first-out order, not last-in, first-out. A graph isn’t the right model for recording a linear history of actions. While a linked list could be used to build a stack, the conventional and most direct choice for undo is a stack because of its straightforward push/pop behavior.