Introduction:
The mode of DNA replication refers to how the two strands of DNA separate and serve as templates for the synthesis of new complementary strands. There are three proposed modes of DNA replication: conservative, semiconservative, and dispersive. The correct answer to this question is option 'B', which states that the mode of DNA replication is semiconservative.

Meselson and Stahl Experiment:
Meselson and Stahl conducted an elegant experiment in 1958 to determine the mode of DNA replication. They used a heavy isotope of nitrogen, N-15, to label the DNA in bacteria and then switched to a lighter isotope, N-14, to observe the replication process. The experiment involved several rounds of DNA replication.

Key steps and observations of the Meselson and Stahl experiment:
1. First, bacteria were grown in a medium containing N-15 as the only source of nitrogen, resulting in all the DNA being labeled with N-15.
2. The bacteria were then transferred to a medium containing N-14 as the only source of nitrogen and allowed to replicate their DNA.
3. After one round of replication, a sample of DNA was extracted and subjected to density gradient centrifugation.
4. If DNA replication was conservative, the original heavy DNA strands would remain intact and form a separate band from the newly synthesized lighter DNA strands.
5. If DNA replication was semiconservative, the resulting DNA would consist of hybrid strands, each composed of one N-15 labeled strand and one N-14 labeled strand. This would result in a single intermediate density band.
6. If DNA replication was dispersive, the resulting DNA would consist of fragments with mixed N-15 and N-14 content, resulting in a blurry band.

Results and conclusion:
The density gradient centrifugation analysis showed that after one round of replication, the DNA formed a single intermediate density band. This result indicated that DNA replication is semiconservative, as predicted by Watson and Crick's model of DNA structure. The N-15 labeled parental strands separated during replication and served as templates for the synthesis of new N-14 labeled strands, resulting in hybrid DNA molecules.

Significance of the experiment:
The Meselson and Stahl experiment provided crucial evidence supporting the semiconservative mode of DNA replication. This finding has significant implications for the accurate transmission of genetic information from generation to generation. It also laid the foundation for our understanding of DNA replication and its role in the inheritance of genetic traits.