Final answer:
The best material to add to cells to radioactively label DNA during replication is radioactive thymine, as it gets incorporated into DNA directly through thymidine triphosphate (dTTP). This allows for detection of DNA synthesis.
Step-by-step explanation:
In order to specifically radioactively label DNA during replication, the best material to add to cells would be radioactive thymine. This is because thymine is one of the four nucleotide bases found in DNA. During DNA replication, cells incorporate the thymine into thymidine triphosphate (dTTP), which then gets incorporated into the newly synthesized DNA strand.
Cultured cells that are incubated with ³H-thymine, a radioactive isotope of thymine, will integrate this labeled base into their DNA, thereby allowing for detection of DNA synthesis and replication. The use of radioactive phosphorous, as mentioned in certain contexts, would indeed label DNA too, but since the specific question asks for the base material, radioactive thymine is the correct answer, not phosphorous which is incorporated into the DNA via the sugar-phosphate backbone.