Which method for enhanced learning involves covering multiple types of problems during one study session?

Spacing refers to distributing study sessions over time rather than cramming. While spacing improves retention and long-term memory, it does not specifically involve covering multiple types of problems in one session. Instead, it focuses on the timing and frequency of study sessions to optimize recall.

Retrieval practice emphasizes actively recalling information from memory, typically through self-testing or quizzes. Although it enhances memory retention, it does not inherently involve varying the types of problems being studied in a single session, which is the defining characteristic of interleaving.

Desirable difficulty refers to challenges that enhance learning by making the material more difficult but manageable. This concept focuses on the level of difficulty in the learning process rather than the organization of content within a study session, such as interleaving different types of problems.