Final answer:
The forgetting hypothesis is the concept that describes how new tasks can interfere with the recall of previously learned ones, necessitating relearning.
Step-by-step explanation:
The concept in question is known as the contextual interference effect, which is related to how memory and learning are affected by performing multiple tasks. The option that suggests the learner forgets task A while performing task B and must relearn or replan task A the next time it is required, is the forgetting hypothesis. This hypothesis relates specifically to retroactive interference, where new information interferes with the retrieval of previously learned information. Desirable difficulties, like the elaboration hypothesis, are also related to memory and learning, but they explain improved memory retention through methodologies different from the forgetting hypothesis. For instance, the elaboration hypothesis suggests that by connecting new information with existing knowledge, one enhances memory retention.