The correct option is B Bacterium
The experiment done by Meselson and Stahl demonstrated that DNA replicated semi-conservatively, meaning that each strand in a DNA molecule serves as a template for synthesis of a new, complementary strand.
The Meselson-Stahl experiment was an experiment which demonstrated that DNA replication was semiconservative. This was realized by using E.coli DNA which had N15 nitrogen isotope (heavier than common nitrogen) and then placing it into N14 media.