In a computer system, which device is functionally opposite of a Key-board?

The correct answer is A. Mouse.

A keyboard is an input device that allows a user to input text into a computer or other electronic device. A mouse is an input device that allows a user to control the cursor on a computer screen. The two devices are functionally opposite in that a keyboard is used to input text, while a mouse is used to control the cursor.

A trackball is an input device that is similar to a mouse, but instead of moving the mouse around on a surface, the user rotates a ball with their thumb. A printer is an output device that prints text or images from a computer onto paper. A joystick is an input