H.261 served as the foundation for later video compression standards such as H.263 and H.264.

• A. H.264
• B. H.320
• C. HIPAA
• D. H.261

Answer: (D)

H.261 introduced the practical, block‑based approach to video coding that later standards refined and expanded. It established the essential ideas you see carried forward: macroblocks as the basic unit, both forward and backward motion prediction, a transform and quantization process (using 8x8 blocks with a DCT-like transform), and entropy coding to create a compact bitstream, along with separate handling of luma and chroma data. Because subsequent codecs built on these same foundations—keeping the block-based structure, motion-compensated prediction, and transform coding while enhancing efficiency and features—the standard that started this approach is the correct foundation for H.263 and H.264. The other options don’t fit: H.264 is a later evolution rather than the origin; H.320 is a broader videoconferencing standard that doesn’t define the compression foundation itself; HIPAA is unrelated to video coding.