In which scenario is a libero allowed to serve?

A libero is a specialized defensive player in volleyball who has specific rules governing their participation in the game. The correct scenario in which a libero is allowed to serve pertains to specific rotational positions as determined by league rules.

In many leagues, including those governed by USAV, the libero can enter the serve position in the rotation as a replacement for a back-row player. This is usually permitted as part of the team's strategy, allowing the libero to contribute to the serve while continuing to fulfill their role in defense. That said, the options available to the libero can vary depending on the specific regulations of a given league or tournament, making it essential to refer to those rules.

The other scenarios listed do not accurately reflect the regulations governing liberos. While the libero can play in various rotations, they are not universally permitted to serve in all situations or simply because they are losing. The option of serving only in the first set is also incorrect, as the ability to serve can occur in any set where the libero is designated to do so according to the applicable rules.