What important rule must be followed when a ball is served?

The requirement for the server to stand behind the baseline is crucial in volleyball as it ensures that the serve is legal. This rule is in place to prevent any advantages gained by standing closer to the net, which would make it easier to deliver a powerful or strategic serve. By serving from behind the baseline, the server not only adheres to the rules but also maintains fairness among players.

Other options, such as needing to serve underhand or jumping to serve, are not mandatory and depend on the player's choice and skill level. Additionally, while calling out the score is part of the game protocol, it is not a strict rule regarding how the serve is executed. Thus, standing behind the baseline is the key rule to ensure a compliant serve.