The right answer is D. Individuals 6 and 14.
Individuals 4 and 6 are not part of the descendence of this family, so union with them does not result in inbreeding and therefore A, B and C are false.
Inbreeding is defined as the result of sexual reproduction between two related individuals (that is, having one or more common ancestors). For a given descendant, it is all the more important that the relationship between parents is narrow. An empirical and approximate concept related to the breeding, as well as to certain matrimonial practices, it was formalized and specified by the genetics with the discovery of the material supports of the heredity.