I would say that adolescence is associated with formal operations. I am not completely sure that this is the correct answer, but it is the answer I would choose. I learned in my psychology class that adolescents begin formal operations, which is where they begin to think more abstractly. Concrete operations are more for younger children who are developing the ability to think about more than one thing. But it does depend on what you have learned in your lesson, because maybe your lesson or text says that adolescence is associated with reversibility. So just double check to make sure. :)
Hope I helped,
Darry