Final answer:
Regular mail service offers various options, including first-class, Parcel Post, Priority, and Express Mail, with Express Mail being the fastest premium service.
Step-by-step explanation:
The regular mail service includes several different types of mail options. These options cater to the varied needs of senders, based on factors such as the speed of delivery, cost, and the nature of the items being shipped. Among the options provided in the question, the correct completion to the sentence 'Regular mail service includes first-class, Parcel Post, Priority, and ____ mail' is Express Mail.
Express Mail is a service offered by postal systems to provide expedited delivery of mail items. It is generally the fastest option provided by the postal service and is considered a premium service. Other mail services like Media Mail are specialized for educational materials, Retail Ground is an economical ground service for non-urgent deliveries, and Certified Mail provides proof of mailing and delivery, often used for important documents.