Is it ensure, insure, or assure?
any member of a group or subgenus (Leucobalanus) of North American ornamental and timber shrubs and trees of the genus Quercus in the beech family (Fagaceae). White oaks have smooth, bristleless leaves, sometimes with glandular margins, and acorns with sweet-tasting seeds that mature in one season. Bur oak and chestnut oak (qq.v.) are members of this group