When driving down the highway, caesar could not but notice the young-looking teenager who seemed to be driving in a reckless manner. he immediately assumed that he was one of those defiant, careless, and irresponsible teens who really did not deserve a license. caesar just made a(n)?

I would say that Cesar made a stereotypical assumption about a teenage driver based not on the facts but on a stereotypical characterization of a person with such an age as a teenager so this is shooting from the hip and could be quite wrong ie the teenager could actually be a careful driver but perhaps timid or nevous if a new driver and that could make him/her look reckless.