Final answer:
The model organism used by Meselson and Stahl was B. Escherichia coli (E. coli), which helped them demonstrate the semi-conservative replication of DNA.
Step-by-step explanation:
The model organism used by Meselson and Stahl was Escherichia coli (E. coli).
Meselson and Stahl experimented with E. coli to understand how DNA replicates. They grew E. coli in a medium containing a 'heavy' isotope of nitrogen (¹5N), which eventually gets incorporated into the DNA.
This model organism helped them demonstrate the semi-conservative replication of DNA, which is now a fundamental concept in molecular biology.