Overlearning: A. refers to continued rehearsal of material after the point of apparent mastery. B. promotes improved recall. C. should not be done, since it leads to increased interference. D. does both a and b.

Overlearning refers to the practice of continuing to rehearse and study material even after the point of apparent mastery. This is often done to increase long-term retention and facilitate improved recall of the material over time.

Now, let's go through the given options: A. Refers to continued rehearsal of material after the point of apparent mastery: This option is directly in line with the definition of overlearning. However, it does not address whether there are any positive or negative outcomes associated with overlearning. B. Promotes improved recall: This option indicates that overlearning has a positive effect on recall, but it does not provide a complete definition of overlearning. C. Should not be done, since it leads to increased interference: This option suggests that overlearning has a negative impact by increasing interference (the inability to retrieve information due to the presence of other, similar information). However, overlearning generally has positive effects on recall, rather than negative ones, so this option is incorrect. D. Does both a and b: This option includes both the definition of overlearning (from option A) and the positive outcome related to improved recall (from option B). This is a combination of the correct elements and is the best answer.

Based on our analysis, the correct answer is Option D (does both a and b) because it encompasses the definition of overlearning as well as its positive implications for recall.