The correct option is D Semi-conservative DNA replication
Meselson and Stahl cultured bacteria in a N15 medium to get the DNA of heavy density. Then, bacteria were shifted to a N14 medium, DNA of replication cycles 0, 1, and 2 was isolated. The DNA was all of intermediate density after one replication cycle and ruled out the conservative replication model, which predicts that both heavy density DNA and light density DNA will be present. This result proved the semi-conservative replication model, according to which all DNA molecules will consist of one N15-labeled DNA strand and one N14-labeled DNA strand. The result of second replication cycle were also in consistent with semi-conservative replication model. Correct option is D.