In volleyball, the following actions are legal:
- Spike the ball: This is a legal and common action in volleyball.
The other options are typically not allowed:
- Touch the net during play: This is a violation and results in a fault.
- Hit the ball twice in a row: This is also usually a violation unless you are blocking or the first touch is a soft hit.
- Cross underneath the net during play: This can be considered a violation if it interferes with the play.
So the correct answer is spike the ball.