Inbreeding is the the matig of individuals closely related in relation to a common ancestry. As the genetic code is inherited from parents, as closely related the individuals are, the bigger is the chance of both individuals having many alleles and genes in common that was passed out through few generations. Therefore, it also increases the chance of both of them having the same deleterious alleles.
So, the correct answer is C. increases the chance that two of the same deleterious alleles occur in the offspring
