This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Dental
The only valid method to assess periodontal regeneration is 
Select and explain. 
A. Radiographic analysis, B. Clinical attachment level, C. Reentry operation, D. Histologic analysis
D: Histologic analysis
Answer: D: Histologic analysis 
Explanation: Clear evidence of a new attachment apparatus can be determined only by histologic analysis of tissue blocks obtained from the healed area.