Which report must be filed for any accident involving conventional ordnance that results in a class A, B, or C MISHAP?

The correct choice is B, as the Explosive Mishap Report (EMR) is specifically designed for documenting accidents involving conventional ordnance that fall into class A, B, or C MISHAP categories. These classes represent varying degrees of severity, with class A being the most severe, typically involving loss of life or significant property damage, while class C pertains to less severe incidents.

The EMR serves to ensure that all necessary details are collected and reported to appropriate authorities for investigation and analysis, helping to improve safety protocols and prevent future incidents. It is crucial in maintaining accountability and compliance within military operations involving explosive materials.

Other options, such as the Incident Mishap Report, Operational Risk Management Report, and Preventive Maintenance Report, do not specifically address incidents involving ordnance and therefore do not fulfill the requirement for reporting accidents related to conventional explosives as defined in the regulations.