Final answer:
In tennis, it is legal for a player to hold up their racquet at the net to block a return, given that they do not touch the ball before it crosses the net, reach over the net to hit the ball, or make contact with the net while the ball is in play.
Step-by-step explanation:
The legality of using a racquet to block a return at the net in tennis depends on whether the player makes contact with the ball or not. If the player's racquet does not touch the ball or if the ball is on the player's side of the net when contact is made, it is generally legal to hold up the racquet for defensive purposes. However, if the racquet touches the ball before it crosses over the net, this constitutes a fault called a 'volley', and the point would be awarded to the opponent.
In the context of a tennis match, players at the net can use their racquet to protect themselves or attempt to return the ball, as long as they respect the rules of the game. If a player reaches over the net to hit the ball before it has a chance to either land or has the potential to land on their side of the court, or if they touch the net with their racquet or body while the ball is in play, that would be a violation.