The sfdx commands that can be used to add testing data to a Developer sandbox are forced: data:bulk:upsert and forced: data:tree:upsert. The forced: data:bulk:upsert command uses Bulk API 2.0 to upsert records from a CSV file into an org1. The forced: data:tree:upsert command uses the SObject Tree Save API to import records from a JSON file that describes objects and relationships2. The forced: data:object:upsert command is not valid, and the forced: data:async:upsert command does not exist.
References:
* Salesforce CLI Command Reference - force:data:bulk:upsert
* Salesforce CLI Command Reference - force:data:tree:upsert

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.