In volleyball, the legality of the actions you've listed is as follows:
A. Spike the ball - Legal, as spiking is a common and permitted action in volleyball.
B. Touch the net during play - Not legal, as players are not allowed to touch the net during play; doing so results in a violation.
C. Hit the ball twice in a row - Not legal, unless it occurs during a single action (like a double contact during a set). Generally, a player is not allowed to hit the ball twice consecutively.
D. Cross underneath the net during play - Generally legal, provided the player does not interfere with the opposing team's play.
Therefore, the correct answer is: A. Spike the ball (This is legal for a volleyball player today.)