The correct answer is D.
The poem discusses thirteen ways of looking at a bird. Each way is different from the others. In this way, Stevens is illustrating how perspective varies from person to person and from moment to moment.
Even the same person can view a bird differently -- how he or she sees it on Monday may not be the same as how he or she sees it on Friday. Furthermore, how one person views it is different from how someone else sees it. Perspective is varied and numerous.
