T-cell receptor diversity is primarily generated by genetic mechanisms that create variability in the receptor molecules.
Gene Rearrangements
T-cell receptor diversity is mainly generated by gene rearrangements, a process that involves the recombination of variable (V), diversity (D), and joining (J) gene segments. This is a key mechanism in generating a diverse repertoire of T-cell receptors.
Thus, the correct answer is \( \boxed{A} \).