In "marriage is a private affair," the young man is an ibo, and the young woman is an ibibo. how are the groups different?

The answer is C) They are different Tribes

In the book, the man and the woman belong to Ibo and Ibibo groups which are two distinct tribes that live in the modern country of Nigeria.
These tribes are found in and around the city of Lagos.

Both groups exhibit slightly different customs and traditions and in the book, a whole story is based around this.
The Ibo are mostly rural tribes who follow old tribal customs and discourage love marriages. Parents in the community usually arrange marriages within the tribe and the children do not have a say.
The Ibibo community however, is more modern and not bound by old traditions.

The book is on the turmoil and confusion that love can create when in between two very distinct African tribes.