D) All of the above.
The contrast between Clayton's initial admiration for Tarzan and his later jealousy and disparagement illustrates the complexities of human nature in multiple ways. It shows that even individuals who may seem kind and noble can be prone to jealousy (A), highlights the coexistence of good and evil within human nature (B), and emphasizes how easily one can fall into negative traits (C). Therefore, all of these aspects together encapsulate the complexity of human emotions and behaviors.