Which mod of the MK-21 delay element is not authorized for use?

The MK-21 delay element is an important component in munitions, and each modification (mod) often comes with specific use instructions and safety protocols. The correct answer indicates that Mod 0 is not authorized for use due to potential safety risks, reliability issues, or changes in operational protocols that have deemed this particular modification unsuitable for current applications.

Identifying which mod is authorized typically involves thorough assessments and evaluations of their performance, safety record, and compatibility with existing systems. In this case, Mod 0 likely failed to meet one or more of these critical criteria, leading to its disqualification from authorized use. Understanding the context of why a specific modification is not authorized is crucial, as it highlights the importance of adhering to updated safety standards and regulations in munitions handling.