Which protection is never displayed on the PFD speed tape?

The correct response is based on the specific nature of the speed protection features displayed on the Primary Flight Display (PFD) of the A320. Alpha floor is a protection mechanism that activates automatically under certain conditions to prevent the aircraft from stalling by increasing thrust to maximum. However, it does not manifest as a visual indication on the speed tape; instead, it triggers the autothrust to deliver maximum thrust when the aircraft reaches a critical angle of attack.

The other options, such as alpha protection, alpha max, and high speed protection, all have visual indicators on the speed tape. Alpha protection and alpha max are associated with maintaining safe flight characteristics, displaying relevant parameters on the PFD to inform pilots of the aircraft's performance limits. High speed protection serves a similar purpose with regard to safe speed limits, providing visual cues to ensure adherence to operational norms. Therefore, among the choices, alpha floor is uniquely characterized by its lack of display on the PFD speed tape, solely operational without a visual cue.