Explanation:

- When studying cultures, anthropologists often identify different levels or stages of cultural development.

- Franz Boas: A pioneering American anthropologist known for his work in cultural relativism. He did not specifically categorize Oceanic cultures into six levels.

- Clark Wissler: Known for the "culture area" concept, focusing on Native American cultures rather than Oceania.

- F. Graebner: A German anthropologist who proposed a theory of Kulturkreise, or cultural circles, and recognized successive levels of culture, including the study of Oceania.

- A.L. Kroeber: An influential anthropologist who worked broadly on culture but didn’t focus specifically on Oceanic cultures.

Correct Answer: F. Graebner