Final answer:
Riley is in the representative stage of creative development by using green for the stem and leaves and red for the flower petals, following real-world color associations.
Step-by-step explanation:
Riley, as described in your question, is at the stage of representative creative development. At this stage, Riley uses colors that are typically associated with real-world objects. For instance, using green for the stem and leaves and red for the petals suggests that Riley is attempting to depict the flower as it appears in reality. Children at this stage tend to focus on realistic colors and shapes that people recognize from the world around them. This contrasts with the other stages such as exploration non-representative, where colors and shapes may not correlate with real objects, or extending basic where more abstract or symbolic elements might be present.