Ans: (D) ammonium chloride
Messelson and stahl performed the following experimental proof for DNA replication. They grew E.Coli in a medium containing 15NH4Cl as the only N2 source for many generations. The result was 15N was incorporated into newly synthesised DNA. This heavy DNA molecule could be distinguished from the normal DNA molecules by centrifugation in a cesium chloride density gradient, Then they transferred the cells into a medium with normal 14NH4Cl and took samples at various definite time intervals as the cells multiplied and extracted DNA that remained as double stranded helicles. The various samples were separated independenlty on CsCl gradients to measure densities.