Under what condition can fish be packaged using reduced oxygen packaging (ROP)?

Reduced oxygen packaging (ROP) is a method used to extend the shelf life of food by reducing the oxygen concentration around the food. This method is important in handling perishable items like fish, as it can help prevent the growth of pathogens and spoilage organisms. The correct answer highlights a crucial safety standard: fish can be safely packaged using ROP when it is frozen before, during, and after packaging.

This condition is vital because freezing fish prevents the growth of harmful bacteria that can thrive in an oxygen-free environment. Freezing preserves the fish and inhibits the development of microorganisms that could lead to foodborne illness. While the other options suggest various methods of handling fish, they do not align with the food safety standards necessary for ROP. For instance, fish that is pre-cooked or marinated may still pose a risk of spoilage if not properly handled or stored under the correct conditions, and packaging fish whenever desired without these precautions could endanger food safety. Thus, the need for freezing before, during, and after packaging ensures that the fish remains safe for consumption while under reduced oxygen conditions.