Explanation:

Input devices are responsible for converting human-understandable data (like text typed on a keyboard, clicks from a mouse, or images from a scanner) and programs into a digital format that a computer can process and understand. Output devices (a, b, d) perform the opposite function, converting computer-processed data into a human-understandable form. Solid state (c) refers to a type of electronic component or storage, not a device category for conversion. Therefore, 'Input' devices are the correct answer.