"If you had told me the truth, I would have helped you" is correct because it follows the proper structure for a third conditional sentence, which is used for hypothetical situations in the past.
The other sentences are grammatically incorrect:
The sentence "If you had told me the truth, I would have helped you" is correct, while the other sentences contain grammatical errors or awkward phrasing.
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.