Two trained introspectionists in different laboratories examined the same stimulus and reported different elementary sensations. Neither could be shown to be mistaken. Why did this kind of disagreement do so much damage to structuralism specifically?

D. Because the observer was the only possible witness to their experience.

The problem was not that observers disagreed, scientists disagree constantly, but that this kind of disagreement could not be settled, since the reporter was the only witness. Contrast the reaction-time work of the same era, where two labs could compare stopwatches and converge. The difficulty was methodological, not a mistake about the mind.