In volleyball, the legal option for a player is:
A. Spike the ball.
Here’s a brief explanation of the other options:
B. Touch the net during play - This is illegal; players are not allowed to touch the net during a play. C. Hit the ball twice in a row - This is illegal unless it is a block followed by a hit or the ball unintentionally hits the player twice. D. Cross underneath the net during play - This is generally not allowed unless it's done safely and without interfering with play.
Therefore, spiking the ball is the only legal action from the options provided.