The right answer is "D.a summary".
Explanation: It is unfeasible to write down everything the teacher says. Consequently, we end up having to select some parts, which we deem most important, to transfer to the notebook, that is, we make a summary. This type of sketch is efficient because through it we are all the time performing the mental work of understanding and absorbing the words to understand what, from everything, is really fundamental.
What becomes a summary is precisely what was processed by the brain as important. The benefit is that, throughout this process, we force ourselves to be very attentive to the explanation and digest all that information, which is worth far more than spending hours reading the textbook.
