To determine which organism is most similar to a polar bear (Ursus maritimus), we can consider taxonomic classification and ecological characteristics.
- Tiger (Panthera tigris): Belongs to the family Felidae, is a big cat, and is not closely related to bears.
- Gray Wolf (Canis lupus): Belongs to the family Canidae and is not closely related to bears.
- Brown Bear (Ursus arctos): Belongs to the same genus (Ursus) as the polar bear and is closely related, sharing a common ancestor.
- Red Fox (Vulpes vulpes): Also belongs to the family Canidae and is not closely related.
Based on this analysis, the organism that is most similar to a polar bear in terms of taxonomy is the Brown Bear (Ursus arctos).