Option D is correct because the provided text states that 'Virtually any part of the plant like leaf, apical meristem, embryo... can be used as a starting material called explant.' It further lists 'root or shoot apical meristems, leaves... and immature embryos' as commonly used explants. Since all the listed options (Shoot apical meristem, Embryo, and Leaf segments) are mentioned as usable parts, 'All of the above' is the correct choice.