The correct answer is A. mouse.
A graphics workstation is a computer that is used for tasks that require high-quality graphics, such as computer-aided design (CAD), computer-aided manufacturing (CAM), and scientific visualization. The three key elements for a graphics workstation are the host processor, display controller, and mouse.
The host processor is the main computer that runs the graphics software. It is responsible for processing the data that is needed to generate the graphics, such as the position of objects, the lighting, and the textures.
The display controller is the hardware that is responsible for displaying the graphics on the screen. It takes the data from the host processor and converts it into a format that can be displayed on the screen.
The mouse is a pointing device that is used to interact with the graphics on the screen. It allows the user to select objects, move them around, and resize them.
The other options are not essential for a graphics workstation. A keyboard is not required, as the user can interact with the graphics using the mouse. A CRT is a type of display that is no longer commonly used. A digitizer is a device that is used to convert images into digital data, but it is not required for a graphics workstation.