The best technique for Sarah to use would be the Feynman technique, because she would be able to test herself by simplifying her explanation of the content. This technique encourages deep understanding by requiring her to teach the concept in her own words, which can highlight areas where her understanding is lacking. By simplifying and explaining the material, she can identify gaps in her knowledge and take steps to address them.
While the SQ3R method and Cornell note-taking method have their benefits, they may not specifically help Sarah pinpoint the concepts she struggles with as effectively as the Feynman technique would. The Feynman technique is particularly beneficial for enhancing comprehension and retention, making it an ideal choice for her goal of improving her understanding of challenging concepts.