Question

____ are placed on dangerous machinery to detect motion, light, heat, pressure, or another stimulus. their presence helps protect operators from injury while working on machines.
labels
mirrors
alarms
sensors

Explanation:

Labels only provide warnings, mirrors aid visibility, alarms alert after a stimulus is detected, but sensors are devices that directly detect motion, light, heat, pressure, or other stimuli, and are used on dangerous machinery to trigger protective measures for operators.

Answer:

Sensors