Which of the following is a common symptom of Attention-Deficit/Hyperactivity Disorder (ADHD)?

Difficulty sustaining attention in tasks or play activities is indeed a common symptom of Attention-Deficit/Hyperactivity Disorder (ADHD). Individuals with ADHD often struggle to maintain focus on tasks for extended periods and may become easily distracted by extraneous stimuli or their own thoughts. This symptom is core to the diagnosis of ADHD and can significantly impact academic performance, social interactions, and overall functioning.

The other options describe behaviors that are not typical of individuals with ADHD. Instead of excessive restfulness, individuals may exhibit restlessness or hyperactivity. While some might achieve high focus on activities they find particularly interesting, this is not a consistent behavior across all tasks. Unusual calmness in social situations is also not associated with ADHD, as individuals usually display impulsivity or may struggle with social cues. Hence, the ability to sustain attention is pivotal in understanding the condition.