People report deepest absorption when a task is neither too easy nor too hard. Why should the sweet spot sit at the boundary between boredom and anxiety?

D. Because attention is fully occupied only when challenge matches skill

Flow is the arousal curve seen from inside: an easy task leaves attention spare and a hard one spends it on the threat, so only a matched challenge occupies it completely. It is also actionable, the way into flow is to adjust the difficulty, not to try harder to concentrate.