Concept:The structural approach in language teaching focuses on teaching sentence patterns in a planned order. It selects and arranges materials step by step.Explanation:This approach is based on the idea that language is made of structures or patterns. Learners master these patterns to form sentences correctly. Teachers choose specific structures and teach them in a graded sequence. Simple patterns come first, then complex ones. This careful selection and ordering of content is called selection and gradation of materials. The approach does not primarily aim at fluency or accuracy alone. Instead, it ensures learners build a solid foundation by learning structures systematically. Hence, the main stress is on how materials are chosen and sequenced for effective learning.Answer:D. selection and gradation of materials