High-Definition Multimedia Interface (HDMI) is the correct answer as it supports both high-definition video and audio through a single cable, which is what the user requires for a 4K monitor and sound system setup. DisplayPort similarly supports high-resolution video and audio, but it was not listed as an option. Video Graphics Array (VGA) does not support audio and also lacks support for high-resolution displays typically associated with 4K monitors. Digital Visual Interface (DVI) typically does not carry an audio signal and wouldn't be suitable for this user's 'single cable' requirement.