This question tests Harry Potter lore about Patronus charms. Harry Potter's Patronus takes the form of a stag, which represents his father James Potter's Animagus form. The stag is also symbolic of protection and guidance in the series. The other options (doe, hare, horse) are other Patronus forms in the series but belong to different characters - doe is Snape's (representing Lily), hare belongs to Luna Lovegood, etc.