B  
At 12 years the mandibular second molar has already finished crown calcification (completed≈7–8 yr) and is erupting; radiographic studies show that at eruption the root is roughly two-thirds to three-quarters of its final length, with an open apex that will not close until about 14-15 yr, matching “crown complete with ¾ root, open apex” [Tsai, PMID 10997244; Feasby, 1981, PMID 6946711].