(C) .jpg
An image file commonly uses the .jpg extension, while .txt is for text files, .exe for programs, and .mp3 for sound files.
Source: Files and Folders, Chapter 3
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The passage explicitly states: "An image file usually has an extension .jpg while a sound file usually has .mp3." Eliminate distractors: .txt = Notepad/text, .exe = executable/program, .mp3 = audio. For MCQs, always support your choice with a brief reason — it shows understanding and can earn full marks.