Which device is commonly used to maintain an electronic appointment book, address book, calculator, and notepad?

The correct answer is a PDA, or Personal Digital Assistant. PDAs were specifically designed to manage personal information and tasks, combining features such as an electronic appointment book, address book, calculator, and note-taking abilities into one portable device. They allowed users to organize their schedules, store contact information, perform basic calculations, and jot down notes, making them highly functional for personal and professional use.

Though smartphones and tablets can fulfill similar roles and have largely supplanted PDAs in terms of technology, a PDA is traditionally recognized for its targeted purpose of personal organization before smartphones became prevalent. MP3 players, on the other hand, are primarily focused on audio playback and do not typically support organizational functionalities like those of a PDA. Thus, the PDA stands out as the correct choice given its specialized design for the tasks mentioned in the question.