The interviewer would like to know if you are the type of person to report unsafe work or if you would rather fit in by protecting your co-workers, even when it's dangerous.
There is a fine balance when answering a question like this because you do not want to seem like an annoying whistle blower but you also do not want to come across as passive when it comes to safety in the workplace.
"If I saw a coworker doing something unsafe on the line, I would speak to them about it in private, immediately. Safety is a major concern in this line of work and I don't take it lightly. If the coworker continued the behaviour, I would then notify my supervisor."