Marking the field as required on the field definition ensures that the field is always populated during record creation or update. This enforces the rule at the database level.
Other Options:
Option A: Setting a default value ensures a value is present initially but does not enforce a value during updates.
Option B: Writing a trigger is less efficient and adds unnecessary complexity.
Option C: Marking the field as required on the page layout is UI-specific and does not enforce the rule during API or code-based operations.
Field Level Requirements