When is a substitution considered legal?

A substitution in volleyball is deemed legal when certain conditions are met. One key requirement is that the substitute enters the match only after the completion of a time-out. This aligns with the rules governing player substitutions, which state that to maintain the flow of the game and ensure proper protocol, a substitute must wait until a time-out has concluded before entering the court. This rule helps in managing the pace of the game and ensures that teams can make strategic changes when it is most appropriate without disrupting the game flow.

The other options do not fully encapsulate the complete conditions for a legal substitution. For example, while having a substitute on the roster is necessary, it alone does not suffice for the legality of the substitution unless the timing is also appropriate. Entering in any order contradicts the regulated procedure for substitutions, which must follow a specific sequence as outlined in the rules. Lastly, the mention of a specific number of substitutions like the team’s 19th substitution does not reflect the general rules governing all substitutions, as there is no stipulation that a substitution can only be executed at a particular point in a match, unless it's related to the limits set for each match or set.

Thus, the requirement that a substitute enters after the time-out has been completed is