One of the main points of the passage is that adults are not infallible in their access to their own thoughts. Rather, they make quick inferences that only seem like direct perception of their thoughts. See line 57: "Thus, they are crucial in creating the illusion of noninferentiality and infallibility."
Early in the passage, the author challenges this idea of infallibility (lines 5-6). The experiment uses children, but the results apply to people in general. The study used children because they are not as able to make quick inferences, making their inferences easier to recognize. This is best expressed by D.