Which word means required or obligatory?

The word that means required or obligatory is "mandatory." This term indicates something that is not just suggested or advised, but rather something that must be done or complied with. In various contexts, such as legal, educational, or organizational settings, when a rule or guideline is described as mandatory, it implies there are consequences for failing to follow it.

The other options convey different meanings: "voluntary" refers to actions that are done by choice, "optional" means that something is left to personal discretion or choice, and "discretionary" also relates to freedom of choice, particularly regarding the use of resources or decision-making. Thus, "mandatory" stands out clearly as the word that signifies a requirement.