Identify the element used by Hershey and Chase to label the protein in their experiment, from the following options :

The correct option is S³⁵.

To understand why sulfur-35 ($S^{35}$) was used to label the protein in the Hershey-Chase experiment, we can break down the molecular differences between proteins and DNA:

1. Chemical Composition of Proteins:
Proteins are composed of amino acids. Some amino acids, such as cysteine and methionine, contain sulfur ($S$) atoms in their chemical structure. However, proteins do not contain phosphorus ($P$). Therefore, by using a radioactive isotope of sulfur, Hershey and Chase could specifically tag the protein coat (capsid) of the bacteriophage without labeling its DNA.

2. Chemical Composition of DNA:
DNA has a deoxyribose sugar-phosphate backbone, meaning it contains a large amount of phosphorus ($P$) atoms. Crucially, DNA does not contain any sulfur. To label the DNA specifically, Hershey and Chase used a radioactive isotope of phosphorus, namely $P^{32}$.

3. Choice of Isotope:
The radioactive isotope of sulfur used in the experiment was sulfur-35 ($S^{35}$), which is an unstable, radioactive isotope that emits beta particles, allowing it to be easily detected. Thus, $S^{35}$ was the perfect marker to trace the behavior and location of the viral proteins during infection.