Which one of the following is not a/an image/graphic file format ?

Concept. A file format is an agreed convention for how bytes are laid out inside a file so that software can read them back, and it is normally signalled by a filename extension and a registered media type. An image/graphic file format is one whose layout encodes picture data: raster formats store a grid of pixels together with header metadata such as width, height and colour depth, while vector formats instead store geometric drawing instructions such as lines, curves and fills. Many familiar three-letter computing abbreviations are not formats at all: some name an interaction style, some a protocol, some a piece of hardware, and none of those defines a byte layout for a stored picture.

Application. Expand each abbreviation and check whether the expansion names an encoding of picture data or something else.

Cross-check. PNG, GIF and BMP each define a concrete byte layout for pixel data and are registered as the media types image/png, image/gif and image/bmp, so any viewer can open a file written in them. Graphical User Interface instead describes how a person operates a program on screen; it is produced by an application’s interface code as it draws windows and responds to pointer events, and it never specifies how a picture is stored on disk.

Result. The term that is not an image/graphic file format is GUI (Graphical User Interface); the other three are all raster image formats.