The correct answer is Cusp of Carabelli.

- The maxillary first molar is unique in having a distinctive anatomical feature called the Cusp of Carabelli.
- This is an additional cusp located on the mesio-palatal surface of the tooth, adjacent to the mesiopalatal cusp.
- It is considered a characteristic trait primarily of the maxillary first molar and is not typically found on other molars.

Let's briefly review the other options:
- Distolingual cusp: While the maxillary first molars do have a distolingual cusp, this feature is also present on other maxillary molars, such as the second molar, so it is not unique to the first molar.
- Oblique ridge: The oblique ridge is a common anatomical feature of maxillary molars in general, formed by the union of the distobuccal and mesiolingual cusps, and is not exclusive to the first molar.
- Prominent buccal groove: This groove exists on all maxillary molars and is not unique to the first molar.

Reference: Orban's Oral Histology, 13th Edition, Chapter on Teeth Anatomy, Page 123