Which type of mail indicates that it requires immediate attention?

The correct answer is Priority mail, as it is specifically designed for faster delivery compared to standard and bulk mail options. Priority mail is used when there's a need to expedite the delivery of items, ensuring they arrive at their destination in a timely manner. This type of mail typically has a delivery window of one to three business days, making it suitable for items that need immediate attention.

Express mail, while also quick, is geared towards the fastest shipping option available, often with next-day delivery, but it might not be as commonly used for less urgent matters that still require faster handling. Standard mail, on the other hand, is the slowest and is generally used for non-urgent correspondence, while bulk mail is used for large volumes of identical mail, often at a discounted rate, but it lacks the immediacy in delivery that priority mail provides.