What must a line judge do during the set in relation to the serve and played ball?

A line judge plays a crucial role in ensuring the proper conduct of the game, particularly during the serve and play of the ball. When it comes to the correct responsibilities assigned to a line judge, indicating whether the ball crosses the net within the antennas is fundamental. This task is vital because it determines if the serve is legal and whether the play can continue. If a served ball lands outside of the antennas, it would be considered out of bounds, and the line judge is responsible for signaling this accurately to maintain the integrity of the game.

In terms of the other choices, notifying the second referee about the Libero's position and checking the service order are important tasks as well, but these responsibilities are typically handled by other officiating roles. The Libero’s tracking and service order verification are essential for the flow of the match, but they fall outside the specific duties assigned to line judges during play. Thus, while all duties are relevant to the overall officiating process, the line judge's primary focus during the set is to indicate the legality of the ball's crossing at the net.