The word "immaculate" means something that is perfectly clean or free from any flaws or stains.
The correct synonym for "immaculate" is "spotless", which also means completely clean.
The words "dull", "healthy", and "modern" are not related in meaning to "immaculate".
The correct answer is "spotless".
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.