The answer is C: the bias present in propaganda material.
If you decide to include propaganda material as a way to illustrate and exemplify the differing ideologies during the times of the Cold War, you should be careful to clearly discern and explain the bias that the propagandistic material you choose to use possesses (propaganda is a form of divulgation material aimed at supporting a claim not based on objective criteria, but rather aimed at persuasion based on ideology). In this way, your assignment will remain objective (which is one of the most important characteristics of all forms of research) whilst at the same time it can be illustrative.