Trenches deeper than what depth must be tested before entry?

Trenches deeper than 4 feet must be tested for hazardous atmospheres before entry, making this the correct answer. The Occupational Safety and Health Administration (OSHA) regulations require that any trench deeper than this depth undergoes testing to ensure that it is safe for workers. This testing is essential to identify potential hazards such as the presence of toxic gases, insufficient oxygen levels, or flammable materials, all of which could pose serious risks to workers' health and safety.

The significance of this regulation lies in the fact that deeper trenches have an increased likelihood of encountering unstable soils and other environmental conditions that could compromise worker safety. Therefore, testing at this depth is a critical precautionary measure mandated by safety regulations.

In contrast, the lesser depths mentioned in the other choices do not require such testing, as they are considered to present a lower risk for hazardous atmospheres.