Which safety measure is required when working at a height of 6 feet or more?

When working at a height of 6 feet or more, a fall arrest system is a critical safety measure. This system is designed to prevent falls from elevated surfaces, which can result in serious injuries or fatalities. According to the Occupational Safety and Health Administration (OSHA) regulations, any work performed at height requires the implementation of appropriate fall protection measures to ensure the safety of workers.

A fall arrest system typically includes components such as safety harnesses, lanyards, and anchor points, all of which work together to catch a worker in the event of a fall. This system is essential when considering the severity of potential accidents that can occur when working from heights, not just for the worker's safety but also for compliance with safety regulations in the workplace.

The other options, while important in various contexts, do not meet the specific requirement for safety at heights. A hard hat protects against falling objects but does not prevent falls. High-visibility vests enhance visibility on a job site, which is important, but they do not provide any fall protection. Ear plugs are used for hearing protection in noisy environments but are unrelated to the hazards of working at height. Thus, the requirement for a fall arrest system is the most relevant safety measure in this scenario.