To implement a process where every time an Opportunity is created, a follow-up Task should be created and assigned to the Opportunity Owner, the most efficient method is to use a Record-Triggered Flow on the Opportunity object.
Option B: Record-triggered flow on Opportunity
Declarative Solution: Salesforce Flows provide a powerful, declarative way to automate business processes without writing code.
Triggering Event: The flow is configured to trigger when a new Opportunity record is created.
Create Task Record: The flow includes an element to create a new Task record.
Set the Assigned To field to the Opportunity Owner.
Populate other Task fields as necessary (e.g., Subject, Due Date).
Efficiency: Flows are more efficient than Apex triggers for simple automation and are easier to maintain.
Best Practice: Salesforce recommends using Flows over Workflow Rules and Process Builders for new automations.
Reference:
Record-Triggered Flows
Flow Builder
Why Other Options are Less Efficient or Incorrect:
Option A: Apex trigger on Task
Incorrect Object: An Apex trigger on Task does not fulfill the requirement, as the trigger needs to be on Opportunity creation.
Code Maintenance: Requires writing and maintaining Apex code, which is less efficient for this use case.
Option C: Auto-launched flow on Task
Incorrect Trigger: Auto-launched flows do not automatically trigger on record creation unless explicitly invoked.
Does Not Meet Requirement: Does not trigger upon Opportunity creation to create a Task.
Option D: Task actions
Ambiguous Term: "Task actions" is not a specific Salesforce feature.
Possible Misinterpretation: Could refer to Workflow Actions, which are being deprecated in favor of Flows.
Conclusion:
Using a Record-Triggered Flow on the Opportunity object is the most efficient and recommended approach.
It leverages Salesforce's declarative tools and aligns with best practices for automation.

A: Custom objects and fields are the fundamental building blocks of the database layer in Salesforce for creating and storing custom data.
C: Roll-up summary fields are a declarative feature used to perform calculations (e.g., sum, count) on child records and display the results on the parent record.
D: Relationships (e.g., master-detail, lookup) are essential for defining how data objects are related to each other.
Why not other options?
B: Triggers are a programmatic solution, which goes against the declarative-only requirement.
E: Flows are used for automation and logic, not for building the database layer.
Salesforce Data Modeling