Option 1: Oblivion -> refers to being forgotten or in a state of forgetfulness, similar to amnesia.
Option 2: Forgetfulness -> means inability to remember things, synonymous with amnesia.
Option 3: Retention -> means the ability to keep or hold information in memory, directly opposite to amnesia.
Option 4: Realisation -> means becoming aware or understanding something, not the direct opposite of memory loss.
Hence, Option 3: Retention -> Amnesia refers to loss of memory, while retention means the ability to preserve and recall information in memory, making them antonyms -> correct