After both hippocampi were removed, patient H.M. practised mirror-drawing daily and got steadily better at it - while insisting at every session that he had never attempted the task before. Why is being better at something you cannot remember learning evidence for more than one memory system?

C. Because the improvement and the recollection came apart.

A dissociation is how neuropsychology argues: one store would have to lose both or keep both, and his error curve fell like anyone else's while his report stayed empty. He was not failing to express an intact memory, he was reliably surprised by the task every session, including when it was described to him.