General opposites:
* Required: This is the most direct opposite, meaning something is necessary and not a choice.
* Mandatory: Similar to "required," this emphasizes that something is enforced and cannot be avoided.
* Compulsory: Another synonym for "required," often used in formal contexts.
* Essential: This implies that something is crucial and cannot be omitted.
* Obligatory: Indicates something is required by duty or law.
Other options:
* Non-optional: This is a more literal opposite, directly negating the concept of being optional.
* Necessary: Similar to "required," this emphasizes that something is essential for a particular outcome.
* Mandatory: Similar to "required," this emphasizes that something is enforced and cannot be avoided.
* Prescribed: This indicates something is specified as a requirement.
The best opposite of "optional" depends on the specific situation and the desired emphasis.