The correct answer is: A. Backup file.
A .bak file is a backup file. It is a file that is created by a program to store a copy of a file in case the original file is lost or damaged. Backup files are often used to protect important data from being lost.
Option B is incorrect because an audio file is a file that contains audio data. Audio files are often used to store music, sound effects, and other audio recordings.
Option C is incorrect because an animation/movie file is a file that contains animation or movie data. Animation/movie files are often used to store videos, movies, and other animated content.
Option D is incorrect because an MS Encarta document is a document that was created using the MS Encarta encyclopedia software. MS Encarta documents are often used to store information about a variety of topics.