An opossum’s ability to play dead when approached by a predator is an example of a what mutation?

It is a behavioral adaptation. 

There are three types of adaptations: 
Structural - body parts that help it survive (e.g. color, size, shape)
Physiological - systems present in an organism that allow it a special feature (e.g. venom, skin color changes)
Behavioral - an action that allows the organism to better survive