The pronoun used in the sentence is 'one' whose corresponding reflexive pronoun is 'oneself'. Therefore, the correct answer to fill in the blank is option A) oneself.
If the pronoun used in the sentence would have been 'him', then the corresponding reflexive pronoun would be option B) himself.
If the pronoun used in the sentence would have been 'them', then the corresponding reflexive pronoun would be option C) themselves.
If the pronoun used in the sentence would have been 'I', then the corresponding reflexive pronoun would be option D) myself.
So, option B, C and D are incorrect.