Final answer:
The grafting technique used for changing the variety of an adult tree by attaching scions of a new variety is called top working (option D). This method allows the use of an adult tree's established root system for the growth of new fruit-bearing scions.
Step-by-step explanation:
The type of grafting used to change the variety of an adult tree by grafting scions of a new variety onto the end of the tree's limb is known as top working. Top working is a method used particularly for older or adult trees where multiple scions from a desired variety are grafted onto the branches of the existing tree, effectively changing the tree's variety once the scions grow and produce fruit. This technique leverages the established root system of the mature tree and offers the benefits of the scion's fruit-bearing characteristics. Moreover, grafting ensures that the vascular cambium of the scion and the stock are in close alignment to facilitate the successful fusion and function as a lateral meristem, which is crucial for the flow of nutrients and water.