Final answer:
An after trigger should be used in Salesforce to automatically create and associate a new opportunity with a newly created account after the account record is successfully saved to the database.
Step-by-step explanation:
The student asked about automating the process of creating a new opportunity in Salesforce whenever a new account is created. To accomplish this, the correct choice would be to write an after trigger. An after trigger is used because it allows the automation to take place after the new account record is successfully saved to the database, making sure that the new opportunity is associated with an existing account. A workflow rule does not have the capability to create new records. A formula field is used for calculations and displaying data, not for creating new records. A before trigger is used to make changes to the record before it is saved to the database, which is not the requirement in this case.