The correct option is B mating of related individuals of the same breed
A pureline refers to a line of descent that consists of homozygous animals that have been raised from closely related parents.
Inbreeding involves mating closely related individuals of the same breed for 4-6 generations. It increases the chances of offspring to receive the same allele from both parents, resulting in accumulating superior genes and eliminating less desirable ones. This process thereby, increases productivity and development of purelines in cattle. Hence option b is the answer.
Out-breeding done between unrelated individuals of same breed or different breeds results in increasing heterozygosity in the population. Hence option a, c and d are not the answer.