Which of the following is a type of still image file?

The answer is .gif because it is a format specifically designed to store still images, as well as simple animations. GIF, which stands for Graphics Interchange Format, supports both static images and short animations that loop continuously, making it versatile for various visual applications, particularly on the web.

In contrast, the other options represent entirely different file types. An executable file, such as .exe, is a program that can be run on a computer and is not related to images. The .mp4 format is used for video files, containing both audio and visual components, and does not pertain to still images. Lastly, a .txt file is a plain text format used for documents that contain no visual imagery. Therefore, .gif stands out as the appropriate choice for a still image file.