Answer: Flashback
Explanation: As the text implies, Harry is looking through the pensieve and gazing at Snape's past. This is clearly a use of flashback, although it is not the actual character, rather, it is an adversary, or in this case Harry. Even if you have doubts that it is not flashback, first, reverse chronology does not apply to this particular scene, as the plot is not unfolding backwards. Some might argue that it is foreshadowing, as later on in book 7 Harry again peers into Snape's memories, but that would not fit, as gazing into Snape's past does not prematurely reveal anything, only that James was abusive towards him. It is also not parallel plots, as it is not another plot. So therefore, flashback would make the most sense.