Double-helical structure of DNA was given by whom?

1. A

   Jacob and Monod

   François Jacob and Jacques Monod explained the operon model of gene regulation in bacteria, not the double helix of DNA. Their Nobel work is molecular genetics of control, not the 1953 structure. This option is not the key.

2. B

   Watson and Crick

   James Watson and Francis Crick proposed the double-helical structure of DNA in 1953, using X-ray data associated with Rosalind Franklin and Maurice Wilkins. Base pairing explained replication. This option is the official key.

3. C

   H.G. Khorana

   Har Gobind Khorana helped crack the genetic code and synthesise oligonucleotides; he did not propose the double helix. This option is therefore wrong.

4. D

   None of the above

   Because Watson and Crick are correctly named, none of the above does not apply. This option is not the key.

Summary. Official key is (b) Watson and Crick. The 1953 Nature paper set out DNA as a double helix with complementary bases. Jacob and Monod worked on the operon; Khorana on the genetic code and synthesis. The stem asks who gave the double-helical structure. Only (b) is that pair.