Final answer:
The most important action when assessing a highly capable third-grade student is to modify the assessments to increase their rigor, ensuring that the student is appropriately challenged and engaged academically.
Step-by-step explanation:
When assessing a third-grade student who recently qualified for gifted services and routinely excels at classroom work and exams without needing to study or prepare, the most important consideration is A. Modifying assessments for the student to increase their rigor. The rationale behind this approach is to provide challenges that match the student's advanced capabilities while promoting further growth and learning. Implementing rigorous modifications can involve incorporating critical thinking and problem-solving tasks that extend beyond the standard curriculum. This approach not only maintains academic interest but also prevents boredom and ensures the continued academic advancement of the gifted student.