Another good explanation from a Stanford researcher: Okazaki fragments are small (100-200 nucleotide) segments of newly synthesized DNA on the lagging strand. The key to this question is an understanding that the Okazaki fragments are newly synthesized DNA. The experiment has been designed as follows: grow cells in medium containing 15N for many generations such that their DNA content contains (almost exclusively) 15N, which was incorporated through successive cell cycles. Then transfer the cells to a medium containing only 14N. When the cells undergo mitosis and replicate their DNA in this new medium, any newly synthesized DNA will incorporate 14N. By definition, Okazaki fragments are newly synthesized DNA, and consequently will contain only 14N, (A).