Final answer:
The finding that indicates the plan for care was effective for a child with ADHD having impaired social interaction is 'D. The child engages in cooperative play with other children', as it directly shows improvement in the area of concern.
Step-by-step explanation:
The question pertains to a child with ADHD (attention deficit/hyperactivity disorder), a neurodevelopmental condition associated with difficulty in maintaining attention and controlling impulses.
The nursing diagnosis relates to impaired social interaction due to the ADHD-related symptoms of aggression and demanding behavior. Among the options given, the most direct evidence that the plan for care was effective would be 'D. The child engages in cooperative play with other children'.
Improvements in social interaction, specifically in the context of cooperation in play activities, directly address the impairment described in the diagnosis. A child showing an improvement in this area is indicative of success in managing the social aspects of their ADHD, which is one of the primary goals for treatment and care of individuals with ADHD.