D is Correct.
In the trigger, we can check if the Lead is owned by a user in the specified subnet (you can determine this based on the User’s owner or some criteria that you define), and then you can update the picklist field to 'Monitor' accordingly.

On the Otherhand, A Workflow Rule is a declarative tool that can be used to update fields based on certain conditions. While this could work in some cases, Workflow Rules are limited to certain field types and can only fire after a record is inserted or updated, which means this would only work for new or updated Leads, not specifically for the "before insert" scenario where the picklist field needs to be set initially.