Semi conservative replication of DNA was first demonstrated in

To obtain heavy density DNA, Meselson and Stahl cultivated E. coli in a 15N medium. 

After that, the bacteria were transferred to a 14N medium, and DNA from replication cycles 0, 1, and 2 were extracted. 

After one replication cycle, all of the DNA was of intermediate density, ruling out the conservative replication model, which predicts that both heavy and light density DNA will be present. 

The semi-conservative replication model, according to which all DNA molecules will include one 15N-labeled DNA strand and one 14N-labeled DNA strand, was proven by this study. The second replication cycle's results were also in line with the semi-conservative replication model. 

As a result, the right answer is 'Escherichia coli.'

The correct answer is  Option D : Escherichia coli