Final answer:
Option (E), V-J rearrangement is a process that occurs in both B cells and T cells as part of their immune receptor development, contributing to the diversity of antigen-binding sites.
Step-by-step explanation:
V-J rearrangement refers to a crucial process during the development of immune cells, particularly B cells and T cells. This genetic recombination involves the combining of V (variable) and J (joining) gene segments to create the diverse antigen-binding sites necessary for the immune response.
Both B cells and T cells perform this process to produce receptors that can recognize and bind a multitude of different antigens. For B cells, this occurs in the light chain of the immunoglobulin molecules, while T cells undergo V-J rearrangement for the T-cell receptor (TCR) alpha chain. Hence, the correct answer to the question 'V-J rearrangement occurs in:' is E. both B cells and T cells.