Marking the field as "Required" in its field definition is the most optimal and robust solution. This enforces the requirement at the database level, ensuring that a value must be provided for this field whenever a record is created or updated, regardless of the entry point—be it the user interface, an API call, or an Apex DML operation. This declarative approach is more efficient and maintainable than custom code and more comprehensive than UI-level settings.

A. Set "Use the first value in the list as the default value" to True: This only applies a default on record creation and does not prevent a user from clearing the value during an update.

B. Write an Apex trigger to ensure a value is selected: This is a programmatic solution for a requirement that can be met declaratively. It adds unnecessary complexity and maintenance overhead.

C. Mark the field as Required on the object's page layout: This only enforces the rule in the user interface for that specific page layout and can be bypassed through other means like the API.

Salesforce Help, "Custom Field Attributes": "Required: Requires users to enter a value in the custom field. This setting is enforced at the data model level, applying to the API and all user interfaces."

Salesforce Object Manager Documentation, "Field-Level Security": "Making a field universally required ensures that no user can save a record without a value for that field."