In volleyball, the legal actions include:
- Spike the ball: This is legal and a common offensive move.
- Touch the net during play: This is illegal; players cannot touch the net while the ball is in play.
- Hit the ball twice in a row: This is generally illegal, except for certain circumstances (like when blocking or in a team's first contact).
- Cross underneath the net during play: This is also illegal; players must not cross the centerline.
Therefore, the only legal action from the options given is to spike the ball.