This organ has separate vascular bundles and, most tellingly, stomates in its epidermis, meaning that it is probably a leaf. Further, there is both spongy and palisade mesophyll, characteristic of a leaf. The unusual thing is that there is a round shape, which is unexpected for the leaf of a flowering plant.
Work Step by Step
To get this answer scan through the chapter, looking for characteristic features of stems, leaves, and roots, making a list for each. Then, using figures in the chapter for help, look for those diagnostic features in the image shown in the question.