Which of the following is a violation of volleyball rules?

In volleyball, a player touching the net while attempting to play the ball is considered a violation of the rules. This is because the net is a barrier that separates the two teams, and interference with it can give an unfair advantage, altering the play. When a player makes contact with the net during a hit or while attempting to block, it's deemed as a fault, resulting in the loss of the rally for the team that committed the infraction.

The other scenarios outlined do not constitute violations under the specified conditions. A player using both hands to set the ball is permitted as long as the action is clean and the ball comes out smoothly without being "held" or "thrown," as per the rules governing setting. A player adjusting their position during a serve is typically allowed, especially if they are within their designated area. The libero, while restricted from attacking the ball above the net’s height in front row positions, can enter the front row in specific situations but cannot attack the ball above the height of the net. Thus, the correct answer highlights the importance of maintaining fair play by adhering to net contact regulations.