All teeth have similar crown-root ratios in this scenario. Which can be used as a direct retainer?

Correct answer: D. Tooth #30 (mandibular 1st molar)

• A. Tooth #24 (mandibular lateral incisor) has a small root surface area, making it a poor choice for direct retainer/abutment compared to a molar.
• B. Tooth #27 (mandibular canine) is a reasonable abutment in some cases, but a molar like #30 offers more favorable retentive surface area for an RPD direct retainer in this comparison.
• C. Tooth #29 (mandibular 2nd premolar) has less surface area/root support compared to a first molar.
• D. Correct. Tooth #30, the mandibular first molar, offers the most favorable crown-root ratio, root surface area, and stability among these choices for use as a direct retainer abutment.