It would be D. Gene Translocation, because chromosome 21 is not being duplicated or cloned like the answer choice B suggests, and it is not be substituted like choice A. I considered C, but decided D would be your best answer despite your suggestion because inversion means reversing something and I know through my knowledge of Biology that this is not the case. The part of the gene (chromosome 21) is being transferred somewhere else and it attaches there (chromosome 14). Nothing is brought back to chromosome 21, making it a one-way exchange that affects both of the chromosomes.
