Final answer:
The Meselson and Stahl experiment showed that the major difference between models was how parental DNA strands are incorporated into daughter strands, supporting the semi-conservative model where each DNA molecule after replication contains one old and one new strand.
Step-by-step explanation:
The primary difference between the proposed models of DNA replication was D. how the parental strands were incorporated into the daughter strands. Meselson and Stahl's experiment provided evidence supporting the semi-conservative model of DNA replication, which suggests that each of the two parental DNA strands acts as a template for new DNA to be synthesized. After replication, each double-stranded DNA includes one parental or "old" strand and one "new" strand.The experiment involved growing E. coli in a medium containing a heavy isotope of nitrogen (15N), which labeled the parent DNA strands. Then, after shifting to a medium with 14N and allowing the DNA to replicate, the extracted DNA, separated by density through ultracentrifugation, formed distinct bands. The intermediate density bands after one replication cycle and the appearance of a light band in addition to an intermediate one after two cycles provided unequivocal support for the semi-conservative replication of DNA.