Which HDMI function allows one AV device to control another connected device?

The correct choice is Consumer Electronics Control (CEC), which is a feature of the HDMI specification that allows one HDMI device to control another connected HDMI device. This function enables interoperability between devices such as TVs, Blu-ray players, and sound systems, allowing users to control multiple devices with a single remote or the control interface of one device. For example, when you turn on a Blu-ray player, it can automatically turn on the TV and switch to the correct input, simplifying the user experience.

Other features related to HDMI, such as Audio Return Channel (ARC), Extended Display Identification Data (EDID), and High-bandwidth Digital Content Protection (HDCP), serve different purposes. ARC is primarily concerned with sending audio from the TV back to an AV receiver, EDID is used for communication between devices to ensure they can support each other's capabilities, and HDCP is a form of digital copy protection to prevent unauthorized duplication of digital content. These functions do not provide control capabilities between devices in the same way that CEC does.