Option 1 -> "From" can be used with time periods, but is less appropriate with the present perfect "has been."
Option 2 -> "Since" is the correct preposition for indicating a point in time with present perfect tense, forming the idiomatic expression "since time immemorial."
Option 3 -> "By" indicates a deadline or method, which doesn't fit the context of duration.
Option 4 -> "About" suggests approximation, which is incorrect for this fixed expression.
Hence, Option 2: since -> "Since time immemorial" is the proper idiomatic expression meaning "from ancient times" or "for as long as anyone can remember," and works correctly with the present perfect tense "has been" to indicate a custom continuing from a distant point in the past to the present -> correct