Question: Which of the following video codecs is NOT explicitly mentioned as registrable for inclusion in 3GP files alongside H.263, H.264, and H.265?
Option 1: MPEG-2 Video
Option 2: VP9 (Video Codec)
Option 3: AV1 (Audio Video Codec)
Option 4: None of the above (all are registrable)
Explanation: The correct answer is VP9 because, based on the provided context, H.263, H.264, and H.265 (HEVC) video codecs are explicitly mentioned as registrable for inclusion in 3GP files. MPEG-2 Video is not listed but is a known standard, making it a plausible distractor. AV1 is an audio-video codec, which might cause confusion due to the 'Audio' part, but it's primarily known for video. VP9, however, is a video codec that is not mentioned in the list of registrable codecs provided (H.263, H.264, H.265, etc.), making it the correct choice as the one NOT explicitly mentioned for registration alongside the others listed.