Each of the following is a requirement for a successful amalgam restoration EXCEPT:

Correct answer: C. An occlusal preparation depth of 1.5 mm is sufficient

• A. ❌ True. Amalgam requires butt-joint (90°) margins. Beveling produces thin unsupported amalgam that chips — bevels belong to gold and composite, not amalgam.
• B. ❌ True. Proper condensation reduces porosity, adapts the material to the walls, and expresses excess mercury from the mix.
• C. ✅ CORRECT — This is the EXCEPTION. Amalgam is a brittle material with low edge strength and requires a MINIMUM occlusal depth of about 2 mm (1.5 mm is sometimes quoted as the absolute floor for pulpal depth in the enamel, but 2 mm of bulk is what the restoration needs to resist fracture). Insufficient bulk is a leading cause of isthmus fracture.
• D. ❌ True. Widening the isthmus beyond a third of the intercuspal distance weakens both the restoration and the remaining cusps.

Note: ⚑ Source-file wording is ambiguous here. Practical numbers: ≥2 mm occlusal depth, ≥1 mm axial depth, 90° cavosurface, no bevels, isthmus ≤ ⅓ intercuspal width.