Answer: a) "early, before their behavior becomes well-practiced and difficult to change"
Explanation: Aggressiveness is defined as the violent nature with the invasion. This kind of behavior should be treated at the start of early age of children so that it does not become a well-practiced piece of nature.
It can become uncontrollable or worse and can become difficult habit to change.It can be controlled by using CBT(cognitive behavioral therapy), medication etc.
Other options are incorrect because elementary school age, middle school age and adolescent age can make the aggressive nature a habit where the child is highly adapted to such behavior.They can also respond with intensity and anger towards treatment. Thus, the correct option is option(a)