Final answer:
The behavior that most clearly indicates an effective treatment plan for a child with ASD is the child holding the parent's hand while walking, as it shows improved social interaction.
Step-by-step explanation:
When evaluating whether a treatment plan for a child diagnosed with an autism spectrum disorder (ASD) is effective, one should look for improvements in behaviors that demonstrate better social interaction and communication skills. Option A, the child playing with one toy for 30 minutes, may indicate improved focus but does not necessarily reflect on social or communication progress. Option B, the child repeating words spoken by a parent, shows some communication skills but is still a relatively basic form of interaction. However, option C, the child holding the parent's hand while walking, shows willingness to engage in social interaction, which is a positive sign of progress in treatment. Finally, Option D, the child spinning around and clapping hands while walking, may be an indication of self-stimulatory behavior which is commonly seen in individuals with ASD and does not necessarily indicate effective treatment. Therefore, the behavior that most clearly indicates an effective treatment plan for a child with ASD is option C, as this demonstrates improved social connectivity and interaction.