What defines the number of pixels that comprise a raster image?

The number of pixels that make up a raster image is defined by its resolution. Resolution refers to the amount of detail an image holds, typically measured in pixels per inch (PPI) or dots per inch (DPI). Higher resolution means more pixels are present in the image, resulting in greater detail and clarity. A raster image is composed of a grid of individual pixels, and the total number of these pixels is determined directly by the image's resolution dimensions. Thus, higher resolution equates to a higher pixel count, making it a key factor in defining the quality and size of the image.

While the other options are important in the context of raster images, they serve different purposes. Bit depth relates to the number of bits used to represent the color of each pixel, impacting color fidelity rather than the actual pixel count. Aspect ratio refers to the proportional relationship between the width and height of an image, which affects its shape but not the total pixel count. Lastly, compression rate involves how much data is reduced to save space, influencing file size but not the inherent pixel composition of the raster image itself. Therefore, resolution is the most relevant term in defining the pixel count of a raster image.