Most commonly submerged tooth is:

 # Most commonly submerged tooth is:

A. Mandibular primary first molar

B. Mandibular primary second molar

C. Maxillary primary first molar

D. Maxillary primary second molar

The correct answer is B. Mandibular primary second molar. 

‘Submerged’ teeth are deciduous teeth, most commonly mandibular second molars, that have undergone a variable degree of root resorption and then have become ankylosed to the bone. This process prevents their exfoliation and subsequent replacement by permanent teeth. After the adjacent permanent teeth have erupted, the ankylosed tooth appears to have submerged below the level of occlusion. This illusion is explained by the fact that there has been continued growth of the alveolar process and also that the crown height of the deciduous tooth is less than that of the adjacent permanent teeth, so that the relative level of occlusion has been changed, not the position of the deciduous tooth. 

The mandibular primary molars are the teeth most often observed to be ankylosed. In unusual cases all the primary molars may become firmly attached to the alveolar bone before their normal exfoliation time.