In volleyball, the only legal action among the options provided is to spike the ball.
- Touching the net during play: This is not legal. A player cannot touch the net while the ball is in play.
- Hitting the ball twice in a row: This is generally not allowed unless a player is blocking (and then it's not considered a hit).
- Crossing underneath the net during play: This can lead to interference and is also not legal.
So, the correct response is: spike the ball.