Final answer:
The radioactive isotope that can be used to label proteins and not nucleic acids is 35S, as sulfur is found in certain amino acids but not in nucleic acids.
Step-by-step explanation:
Among the options given, the radioactive isotope that can be used to label proteins and not nucleic acids is 35S (sulfur-35). This is because sulfur is found in the amino acid's methionine and cysteine, which are components of proteins, but sulfur is not typically found in nucleic acids. Hershey and Chase used 35S to label the protein coat of phages, while they used 32P (phosphorus-32) to label the nucleic acids (DNA) since phosphorus is a key component of the DNA backbone. Radioactive isotopes like 14C (carbon-14), 3H (tritium), and 15N (nitrogen-15) are also commonly used as tracers in biological systems, but for the specific task of labeling proteins without labeling nucleic acids, 35S is the correct choice.