Which action will invalid a fair catch signal?

A fair catch signal can be invalidated based on specific actions that break the rules regarding how the signal should be made. Touching the ground before signaling is significant because the rules specify that the signal must be done while the player is in a standing position and in a position to catch the ball. If the player touches the ground before signaling, it shows that they are not in the proper stance or readiness to execute a fair catch, thus invalidating the signal.

The other actions mentioned in the options do not carry the same consequences for the validity of a fair catch signal. For instance, waving an arm without signaling doesn't constitute an official fair catch signal; it simply doesn't meet the criteria for a valid signal. Making a sound while signaling can be seen as part of the signal if it's done correctly, but if the signal itself is done properly, the sound will not negate the signal. Similarly, not being fully in the neutral zone is more about positional play rather than the act of signaling for a fair catch. Thus, touching the ground prior to signaling is what directly invalidates the fair catch signal according to NFHS rules.