Final answer:
Terminal deoxynucleotidyl transferase (TdT) introduces nucleotides to a free 3' OH end in a template-independent manner after the cleavage of the interstrand hairpin, contributing to immune system diversity.
Step-by-step explanation:
The question asks what introduces nucleotides to a free 3′ OH end created upon cleavage of the interstrand hairpin in a template-independent manner.
The correct answer is A. TdT, which stands for Terminal deoxynucleotidyl transferase.
TdT is an enzyme that adds nucleotides to the ends of a DNA molecule without the need for a template, which is particularly important in the process of V(D)J recombination during immune system development, where it adds nucleotides to the ends of cleaved genes to create diversity in antigen receptors.