Which of the following devices would MOST likely require a serial adapter to be installed on a modern PC?

Traditional external modems connect via RS-232 serial ports (COM ports), which are absent on most modern PCs. To use such a modem, a USB-to-serial or PCIe serial adapter would need to be installed. This makes external modems the device most likely to require a serial adapter on contemporary hardware.

TV tuners connect via USB or PCIe slots, which are standard on all modern PCs - no serial adapter is needed.

Modern mice connect via USB or wirelessly via a USB receiver - serial mice are obsolete and not the device most likely to require a serial adapter today.

Modern keyboards connect via USB or wirelessly - serial keyboards are obsolete and not the most likely device to require a serial adapter in current use.