Choice D is the best answer. The largest single group of participants represented on the graph is composed of those who remembered nothing, as indicated by the bar that is farthest to the right. Why approximately 40% of participants could not remember the statements or the folder locations isn't explained by the description of the fourth experiment in the fifth paragraph of the passage. Therefore, the most likely explanation for the findings regarding the largest single group of participants represented on the graph is that there isn't enough information to determine the cause of the results for those participants.
Choices A, B, and C are incorrect because these speculations aren't supported by the passage. There isn't enough information provided about the fourth experiment to know whether the participants who could remember nothing focused on remembering the folder locations (choice A), attempted to remember the statements and the folder locations (choice B), or didn't attempt to remember any specific pieces of information (choice C).