Explanation:

- Ao Nagas: The Ao Nagas are indeed a tribe. They are one of the Naga tribes and predominantly found in Nagaland, India.

- Khasis: The Khasis are a significant tribe in Meghalaya, India. They have a distinct culture and language.

- Kulin: The Kulins are not a tribe. They refer to a certain caste or social class within Bengali society known for their high status, not a tribal community.

- Ho: The Ho people are a tribal group residing primarily in the Indian states of Jharkhand and Odisha.

The correct option identifying what is not a tribe is:

- Option 3: Kulin

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