Which word describes something that is necessary or required?

The term "requisite" signifies something that is essential or required in a particular context. It indicates that a certain condition, item, or quality must be present for something to be complete or successful. In various fields such as academics, employment, or even everyday scenarios, a requisite can refer to prerequisites needed to embark on a process, such as courses required before taking advanced classes or skills necessary for a job.

In contrast, the other choices indicate a lack of necessity. For example, "optional" suggests that something is not necessary and may be chosen at one's discretion. "Superfluous" refers to something that goes beyond what is needed; it is excessive. "Extraneous" describes something that is irrelevant or unrelated to the subject at hand, making it unnecessary. Thus, "requisite" stands out as the word that accurately conveys the notion of necessity or requirement.