What is an example of an illegal block?

An illegal block occurs when a player attempts to block an opponent's serve, which is against the rules in volleyball. The serve is considered a distinct phase of play, and blocking is only allowed during a rally when the ball is in play after the serve has been successfully executed. Blocking a serve interferes with the intended flow of the game, as the receiving team must play the ball after the serve, rather than attempting to block it.

Blocking a spike is a legal maneuver that occurs during a rally when both teams are engaged in returning the ball to one another. Similarly, blocking during an attack is allowed as long as it is done correctly after the ball has crossed the net and is in play. Therefore, blocking a serve stands out as an illegal action, making it the correct answer to this question.