What ensures digital receivers not controlled by the cable operator receive EAS messages?

The correct answer highlights the role of Program and System Information Protocol (PSIP) messages in ensuring that digital receivers not controlled by the cable operator can receive Emergency Alert System (EAS) messages. PSIP is a set of metadata standards used in digital television, and one of its functions is to relay important information about the service that is being received, including notifications about emergency alerts.

When a digital receiver is tuned to a broadcast channel, it interprets the PSIP data, which includes EAS messages when they are transmitted. This capability allows receivers that are not managed or directly influenced by the cable operator's decision-making to still obtain crucial emergency information, thereby enhancing public safety without requiring a direct, proprietary control from the cable operator.

Other options, such as a dedicated satellite link or hardware upgrades, do not directly pertain to the standardized method for disseminating EAS messages across all types of receivers. A manual tuning switch also does not relate to the automatic relay of alerts, as it simply allows for user-controlled channel selection. Thus, PSIP messages represent the best method for ensuring that all compliant digital receivers, regardless of operator control, can access emergency alerts.