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."
Arun’s present age in years is 40% of Barun’s. In another few years, Arun’s age will be half of Barun’s. By what percentage will Barun’s age increase during this period?