To ensure process compliance where cases cannot revert to an earlier status and certain fields are required based on specific case criteria, the recommended solution is:
Configure validation rules with help text (A). Validation rules enforce data integrity and business processes by preventing users from saving records if certain conditions are not met. For instance, a validation rule can prevent the status of a case from being changed back to an earlier status and can conditionally require fields based on other field values on the case.
Dependent picklists (B) ensure field values depend on another field's value but do not enforce the inability to revert status. Approval processes (C) are used for step-by-step record approval and are not designed to prevent status reversion or conditionally require fields. Making fields required on the page layout (D) ensures they are always required when accessing the record through that layout but does not enforce conditional requirements based on other criteria.
Reference for creating validation rules in Salesforce:
Validation Rules: https://help.salesforce.com/articleView?id=sf.validation_rules.htm&type=5