What is the MINIMUM length of gutta-percha that should remain apically when preparing a post space?

Correct answer: B. 4 to 5 mm

• A. 8 mm would leave inadequate space for a post of useful retentive length in most roots.
• B. Correct. A minimum of 4–5 mm of apical GUTTA-PERCHA must remain to preserve the apical seal. Removing more risks reinfection of the periapex and defeats the endodontic treatment. This creates the fundamental clinical tension in post design: the post wants length for retention (ideally equal to the crown height or two thirds of the root length), while the apical seal demands its 4–5 mm. Where the two cannot both be satisfied, the SEAL wins.
• C. 3 mm is below the generally accepted safe minimum.
• D. 2 mm is insufficient — leakage rates rise sharply below the accepted minimum.