“When thinking” is the right answer (the definition of retrospect is “a survey or review of a past course of events or period of time”. So “when thinking “ is most likely the right answer.
The lines which are a flashback here are: She thought of her last day in middle school, seven years ago, when Sarah had given her an iPod. She had felt ecstatic that day when she realized she would be able to listen to her favorite songs and fend off comments from her cousins about how little she knew about music.
Answer: Precise combination of rapid reading and eye movements is the key when it comes to reading fast.
Hope that helps :)
