Final answer:
The customer should use a scheduled monthly automation with an import activity and triggered send to automate the monthly promotional email.
Step-by-step explanation:
The method that should be used to automate the customer's monthly promotional email is option C: Create a scheduled monthly automation that includes an import activity and triggered send.
By creating a scheduled monthly automation, the email will be sent automatically on the 15th day of each month. If the 15th falls on a Saturday or Sunday, the email will be sent on the prior Friday, as per the customer's requirements.
Using an import activity ensures that the audience file uploaded by the customer will be included in the email send, while a triggered send ensures that the email is deployed immediately after the import activity is completed.