Final answer:
When a CBRN attack is imminent or in progress, the typical response is to declare MOPP level 4, which is the highest MOPP level, requiring full protective gear to ensure maximum protection against contamination.
Step-by-step explanation:
The MOPP (Mission Oriented Protective Posture) level that will be declared when a CBRN (Chemical, Biological, Radiological, and Nuclear) attack is imminent or in progress depends on the specific threat and the protocols established by the military unit or command. However, typically in such situations, the command will declare MOPP level 4. This is the highest MOPP level and requires personnel to wear full protective gear including overgarments, footwear covers, protective mask with hood, and gloves to provide maximum protection against contamination. Each MOPP level corresponds to a different state of readiness and the gear to be worn, with level 4 being a full-response scenario aimed at ensuring the highest level of protection for military personnel against CBRN threats.