Which of the following is the method developed by Matthew Meselson and Franklin Stahl to separate heavy DNA with N${}^{15}$ from DNA with N${}^{14}$, for providing evidence for semi-conservative replication of DNA?

The correct option is A Density gradient centrifugation
The method developed by Matthew Meselson and Franklin StahI to separate heavy DNA with N${}^{15}$ from DNA with N${}^{14}$ was density gradient centrifugation. it separates the DNA molecules in different layers according to their density. The sample is placed on a preformed gradient cesium chloride. Centrifugation separates the macromolecules in different bands of density gradient which can be collected as a pure fraction. Option B is correct answer.