Among scientists, the discovery of the double helix structure of DNA and the genetic code it incorporates is widely regarded to be one of the most significant scientific discovery of the twentieth century.

In the given sentence, the verb is "regard", which takes the proposition "as". So the correct construction should follow " ... to be regarded as..." ; secondly, the phrase "one of the" always takes a plural noun, as it essentially implies one of the many. 

So the correct sentence should be "Among scientists, the discovery of the double helix structure of DNA and the genetic code it incorporates is widely regarded as one of the most significant scientific discoveries of the twentieth century. ", making option B the correct choice.