Hard-feeling practice often beats easy-feeling practice long-term

Correct: C — Often improve long-term retention and transfer relative to easier conditions

Bjork (1994) collected evidence that interventions which slow acquisition — spaced rather than massed practice, interleaved rather than blocked practice, varied rather than constant practice, retrieval tests rather than restudy — typically yield superior long-term retention and transfer. The label 'desirable difficulty' captures the counterintuitive split between training-phase performance and post-training learning: the conditions that feel hardest are often the most productive. The framework anticipates the classroom and athletic-training literatures: shuffling problem types, leaving gaps between sessions, and quizzing through the gap nearly always wins on the test that matters.