Why was SHA-3 standardised?

Answer

SHA-2 is structurally similar to SHA-1, so a different construction was wanted in reserve

Options - A. SHA-2 is structurally similar to SHA-1, so a different construction was wanted in reserve - B. SHA-2 had been broken by a practical collision attack - C. SHA-2 could not produce digests longer than 256 bits - D. SHA-2 required the whole message to be buffered before hashing could begin Why - A. Correct — SHA-3 is a complement to SHA-2, not a replacement. A weakness found in one construction leaves the other available. - B. It has not been. NIST still considers SHA-2 secure for general use, which is exactly why SHA-3 is described as a complement. - C. SHA-384 and SHA-512 have existed since FIPS 180-2 in 2002. Digest length was never the constraint. - D. Backwards: preserving SHA-2’s ONLINE nature — processing 512- or 1024-bit blocks as they arrive — was a stated requirement ON the SHA-3 candidates.

Stallings & Brown, Computer Security 5e, ch2 §2.2; ch21 §21.1; ch23 §23.2