When is a fair catch signal considered invalid?

A fair catch signal is considered invalid when it does not meet the requirements of a valid signal outlined in the rules. For a fair catch to be valid, the player must raise one arm above their head and wave it from side to side. If these criteria are not fulfilled—such as if the player fails to clearly signal or executes a motion that doesn't conform to the standard for a fair catch—the signal cannot be recognized by officials. Therefore, the play will continue as normal without the benefit of a fair catch.

In this context, other options do not directly relate to the validity of the signal itself. For instance, a thrown ball does not influence the legitimacy of a fair catch signal made by a player receiving a kick. Similarly, while the whistle indicates the stoppage of play, it does not affect whether a fair catch signal was valid or not. Lastly, the status of the play clock is unrelated to whether a player properly executed a fair catch signal. Thus, the focus on the specific requirements for signaling makes the second option the accurate choice.