For the requirement to automatically create an Invoice record with associated Invoice_Line_Item__c records for each product on an Opportunity when it closes as won, the best option to implement this is:

**D. Use an Opportunity that calls a Flow.**

Here's a step-by-step explanation for choosing this option:

- **Flow**: A Flow in Salesforce is a powerful tool for automating complex business processes without writing code. It can handle logic that includes creating and updating multiple records based on specific conditions. For this scenario, a Flow can be designed to trigger when an Opportunity is marked as 'Closed Won'. The Flow would then create an Invoice record and iterate through each Opportunity Product associated with the Opportunity to create corresponding Invoice_Line_Item__c records. This ensures that all necessary data from the Opportunity and its products are correctly transferred to the new Invoice records.

- **Triggering the Flow**: The Flow can be triggered automatically from the Opportunity object. This can be configured using Process Builder or a Flow Trigger (depending on the Salesforce setup), which listens for the specific condition of an Opportunity closing as won.

The other options are not as well-suited for the described requirements:

- **A. Use Einstein Next Best Actions.**
- Einstein Next Best Action is generally used for suggesting actions to users, not automatically executing data-intensive back-end processes like creating records based on business logic.

- **B. Use an Opportunity Approval process.**
- An approval process is used for gaining approval on records before proceeding with further actions. While it could be part of a larger workflow, on its own, it doesn’t handle the creation of new records based on the approval of an opportunity.

- **C. Use a custom button on the Opportunity.**
- A custom button could initiate a Flow or other process but would require manual intervention by the user to click the button. This does n...