Which one of the following is one of the two groups of confined spaces?

The correct choice highlights an essential classification within the context of confined spaces. Permit required confined spaces are defined as locations that have specific hazards associated with them, such as toxic atmospheres, insufficient oxygen, or the potential for engulfment. These spaces necessitate a permit system to ensure that safety protocols and precautions are followed before entry.

The reason this classification is crucial is that it indicates the need for a higher level of safety measures. Workers entering these areas must be trained, equipped with proper safety gear, and follow stringent procedures to mitigate risks. The existence of hazards necessitates that a permit be obtained to assess whether conditions are safe for entry.

In contrast, categories like below grade, above grade, and mobile confined spaces may describe different physical characteristics or locations of confined spaces, but they do not necessarily address the inherent risks associated with the working environment, which is a critical aspect of confined space safety. Thus, while other options pertain to the general characteristics of confined spaces, only permit required confined spaces directly address the safety protocols that must be adhered to for safe entry and operation.