The correct option is B False
15N is not a radioactive isotope. 15N and 14N were separated in Meselson and Stahl’s experiment by centrifugation in a cesium chloride (CsCl) density gradient. The heavier DNA molecules were the ones into which 15N had got incorporated, while the lighter DNA molecules were the ones which had 14N in them. As a result of ultracentrifugation, the heavy DNA molecules would settle at the bottom while the lighter DNA molecules would remain on top.