The boy born in class B will marry a girl of class A. So, the daughter will belong to class A. Thus, (a) is false.
Since the boy's daughter will belong to class A, she would marry a boy of class B and so the son-in-law would be feom class B. So, (b) is false.
The boy, born in class B, will have mother born in class B and father born in class A. So, his uncle (i.e., mother's brother or father's brother) can be from class A or B.
So, (c) is true.