Final answer:
The Meselson and Stahl experiment used radioactive isotopes, specifically the heavy isotope of nitrogen 15N, to label the DNA of E. coli bacteria to study DNA replication. so, option 1 is the correct answer.
Step-by-step explanation:
In the Meselson and Stahl experiment, the bean plants (Vicia faba) were not labeled for the purpose of making autoradiographs of chromosomes; rather, it was E. coli bacteria that were used in their study to understand DNA replication. The E. coli were grown in a medium containing a "heavy" isotope of nitrogen, specifically 15N. After several generations, when the DNA was fully labeled with 15N, the bacteria were transferred to a medium with the normal isotopic form of nitrogen, 14N, and were allowed to grow for one generation.
They used autoradiography to visualize the DNA of the E. coli after it had been extracted and separated by density in a cesium chloride gradient. This technique was crucial to support their hypothesis for semi-conservative DNA replication. Therefore, the correct answer to the student's question would be the bean plants were labeled with radioactive isotopes.