What must a player do to ensure a legal fair catch signal?

To ensure a legal fair catch signal, a player must make a lateral waving motion with one arm at full length above their head. This specific signaling method is recognized within the rules of the game to indicate a fair catch. By using one arm and waving laterally, the player clearly communicates to all on-field participants that they are not attempting to advance the ball and are instead signaling for a fair catch. This is essential for player safety, as it alerts the kicking team that the returner will not attempt to run the ball after making the catch.

The other options do not accurately reflect the required movements for signaling a fair catch as outlined in the rules. For example, simply raising one arm or waving both arms does not conform to the needed mechanics for a legally recognized fair catch signal. By requiring a specific motion and position, the rules ensure clarity and safety during the game.