Which of the following is true of H.261, H.263, and H.264?

A.
They are all audio codecs.

B.
They are all HD video codecs.

C.
They all contain error feedback mechanisms.

D.
They were all established by MPEG.

E.
They all support Iframes, P-frames, and B-frames.

Explanation:
261, H.263, and H.264 all contain error feedback mechanisms, primarily through the use of Long-Term
Picture (LTRP) frames. LTRP frames are a type of I-frame. I-frames, which are also known as key
frames, are used as a reference for other frames.
Video formats, which are commonly referred to as codecs, specify how video compression is to be handled.
H.261, H.263, and H.264 are all video codecs; however, not all of them are high definition (HD) video codecs.
Of these codecs, only H.264 supports HD. Most Cisco IP video endpoints use H.264 by default.
H.261, H.263, and H.264 are not audio codecs. Examples of audio codecs include G.711, G.729, and
Advanced Audio Coding Low Delay (AACLD), which is also known as the MPEG-4 Low Delay Audio Coder.
H.261, H.263, and H.264 all support Iframes and P-frames, but only H.263 and H.264 support B-frames; H.261
does not. Iframes are the largest and most important type of frame. P-frames, or predictive frames, are smaller
because they store only the information that has changed from the preceding Iframe. B-frames, or bidirectional
predictive frames, are even smaller because they contain only the image differences between the preceding
Iframe and the subsequent P-frame.
H.261, H.263, and H.264 were not all established by the Motion Pictures Expert Group(MPEG); only H.264 was
established by MPEG. H.261 and H.263 were established by the International Telecommunication Union
Telecommunication Standardization Sector (ITUT).

Cisco: Cisco Video and TelePresence Architecture Design Guide: Basic Video Concepts: Video Compression
Formats