Answer:
Speakers.
Explanation: Cause they only output sound, and are devices, further more they are not used to output images, like how could speakers output images using sound? They can't.
Ask someone who knows about this stuff this is weird I would say look it up maybe
A Human-Machine Interface (HMI) is a graphic terminal that allows the robot operator to control, monitor, and collect data from the robot system.
(https://www.robots.com/faq/what-is-a-hmi)