Which option would best describe a raster image format?

A raster image format is best described as being composed of pixels. Raster images, also known as bitmap images, are made up of individual dots of color (pixels) that together create an overall picture. Each pixel in a raster image has a specific color value, and the resolution of the image is directly tied to the number of pixels it contains. When a raster image is scaled up or down, the individual pixels become more or less apparent, which can lead to a loss of quality or pixelation at larger sizes.

In contrast to this, other formats mentioned are either based on vector graphics, which rely on paths and mathematical equations to create images that can be scaled infinitely without losing quality, or exhibit characteristics that aren’t typical for raster images—like resolution independence or being primarily used for illustrations. Thus, the defining characteristic of a raster image format is its composition of pixels.