What type of mail piece would most likely require a tracking number?

The correct choice is a priority package, which is designed to provide faster delivery service than standard options. Priority packages typically come with enhanced features such as tracking numbers, allowing both the sender and recipient to monitor the package's journey from origin to destination. This added layer of service is particularly important for time-sensitive or valuable items which need accountability during transit.

Standard letters, postcards, and items sent with first-class stamps usually do not include tracking unless extra services are purchased. These options generally provide basic mailing services without the same level of detail and assurance regarding the package's movement and delivery status that tracking offers. Thus, priority packages stand out as the type of mail piece that most frequently necessitates a tracking number for effective monitoring.