Answer:
Suggestibility.
Explanation:
'Suggestibility' is demonstrated as the one of the seven sins of memory that involves inclusion of false suggestions in our memory. It is described as the acceptance of overt information/suggestion. Since our memory is often affected by our past memories and even a slight emphasis is laid on some specific attributes, the individuals accept them in their memory without judging its validity.
As per the question, the 'suggestibility' flaw of memory is explained with the given fault as the slight emphasis on the a past memory(robbery event by asking about noticing the robber's hat) the witnesses includes this false suggestion in their memory(as the witnesses report that the robber wore a hat even though he had not worn a hat at all) without checking its credibility. Thus, it exemplify 'suggestibility' sin of memory.