Option 1 -> Notorious means famous for something bad; similar negative connotation, not opposite.
Option 2 -> Blatant means obvious and shameless; this is actually a synonym of flagrant, not an antonym.
Option 3 -> Unobtrusive means not conspicuous or attracting attention; the opposite of flagrant.
Option 4 -> Adjacent means next to something; unrelated to the meaning of flagrant.
Hence, Option 3: Unobtrusive -> Flagrant means conspicuously offensive or glaringly obvious, while unobtrusive means subtle and not attracting attention, making them perfect antonyms -> correct