Double helical model of DNA molecule was proposed by -

Correct option is B

The correct answer is (b) Watson and Crick.
· James Watson and Francis Crick are credited with the discovery of the double helix structure of DNA in 1953.
· They proposed that DNA is composed of two strands that coil around each other to form a double helix, with the bases adenine, thymine, guanine, and cytosine pairing in specific ways (A-T and G-C).
Other options explained:
· Khorana: Har Gobind Khorana contributed to the understanding of the genetic code and was involved in nucleotide synthesis, but he did not propose the double helix model.
· Morgan: Thomas Hunt Morgan is known for his work on the role of chromosomes in heredity, particularly using fruit flies, but not for the DNA structure.
· Nirenberg: Marshall Nirenberg is famous for cracking the genetic code, demonstrating how sequences of nucleotides in DNA and RNA specify amino acids, but he did not propose the double helix model.