The word "scissors" is always plural, but when referring to "a pair of scissors," it is treated as singular.
The correct phrase is:
"A pair of scissors is blunt."
The correct sentence should be:
"A pair of scissors is blunt."
Thus, the correct answer is Option D ("A pair of scissors is blunt").
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.