In volleyball, the only legal action among the options provided is:
- Spike the ball: This is a legal and common action during gameplay.
The other actions are illegal:
- Touching the net during play: This is not allowed.
- Hitting the ball twice in a row: Typically considered a fault unless it occurs during a block or if the first hit was a legal play (like a set).
- Crossing underneath the net during play: This is also not permitted as it can interfere with the opposing team's play.
So the legal action is "spike the ball."