In American Sign Language (ASL), an "unmarked handshape" typically refers to handshapes that do not have any additional modifiers or variations. Common unmarked handshapes include the open hand (A), closed fist (B), and others like flat hand (S).
Since you mentioned "A," "B," and "S," I assume you are listing potential options. However, without having access to the specific list or context in which these handshapes are being used, it is challenging to identify which one is not an unmarked handshape definitively.
If you have any other context or specific handshapes in mind, please provide that information for a more accurate answer.