Final answer:
Building on what students already know while adding new concepts represents teaching within students' zone of proximal development. So, the correct answer is option d.
Step-by-step explanation:
The instructional approach that best represents teaching within students' zone of proximal development is Building on what students already know while adding new concepts. This approach involves connecting new information to prior knowledge and skills that students have already acquired.
By building on what students already know, teachers can provide instruction that is challenging but still within their capabilities, allowing them to make progress and further develop their understanding. So, the correct answer is option d.