All markings should follow the international color code for ___________.

The correct choice is associated with the need for standardized marking to ensure safety and compliance when identifying underground facilities. The international color code for underground facilities is crucial for various reasons, including preventing accidental damage to utilities, enhancing public safety, and facilitating efficient excavation processes.

Using consistent color codes allows workers to quickly identify the type of utility present underground, such as gas, water, electricity, or telecommunications. This standardization helps to reduce the risk of accidents during excavation or construction, as personnel can recognize and respect the markings that indicate the location of these facilities.

The other options, while they may involve markings, do not specifically address the universally recognized system of colors applied to underground utilities. For instance, mining and drilling often pertain to above-ground operations, while piping fixtures focus on in-plant installations, and subterranean structures can cover a broader range of underground types beyond strictly facilities. Thus, the emphasis on underground facilities aligns directly with the context of the international color code used in the field.