Final answer:
Brian exhibits symptoms of Attention-Deficit/Hyperactivity Disorder (ADHD), such as inattention, executive functioning difficulties, impulsivity, and hyperactivity, which affect his school performance and social interactions.
Step-by-step explanation:
Brian most clearly exhibits symptoms of Attention-Deficit/Hyperactivity Disorder (ADHD), which is a neurodevelopmental disorder characterized by difficulty in maintaining attention and controlling impulses. Symptoms of ADHD, which is more prevalent in males than females, include inattention (lack of focus), executive functioning difficulties, impulsivity, and hyperactivity beyond what is characteristic of the normal developmental stage. It is also important to note that individuals with ADHD may have comorbid conditions, such as depression or obsessive-compulsive disorder (OCD), and may exhibit challenges in educational settings due to frustrations, distractions, and difficulties in following instructions.