The subject "one" refers to a general person, and when using a reflexive pronoun, it should be "oneself."
The correct sentence should be:
"One should keep oneself ready for emergency."
Thus, the correct answer is (C) "One should keep oneself ready for emergency."
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.