Option 1 -> "In memory" is not a standard phrase used when reflecting on past decisions.
Option 2 -> "In recollection" is grammatically awkward and not commonly used in this context.
Option 3 -> "In retrospect" is the correct idiomatic expression meaning "looking back" or "upon reflection."
Option 4 -> "In reminiscence" is not a standard phrase for expressing hindsight reflection.
Hence, Option 3: retrospect -> "In retrospect" is a common English idiom used when reconsidering past events with the benefit of hindsight, perfectly fitting the sentence's meaning of reflecting on how one should have acted differently -> correct