it refer to what? any medical test ? or else?

A and B:
indicate that there is one when it is not => it refers to what. IT suppose to refer one but cannot.

One's or one can refer to one but no one else. This rule strictly followed in British English but not in American English. I believe this.

It is a Pronoun and if a Pronoun can refer to more than one noun in the sentence, it is ambiguous. However, ambiguity does not always mean the sentence is wrong. If you have eliminated all the must-eliminate errors (grammar errors for example) and all you have left is an ambiguity, it may still be the correct answer choice even though it should be wrong, technically. If the ambiguity is obvious (ie we do not know what the pronoun refers to) then the chances are very high that there s a better answer choice, but if the ambiguity is only a little but ambiguous (ie it is mostly clear what is being referred to)then such an ambiguity may stand.

As for the 'one': this 'pronoun' can only refer to itself. For example: One can see the building when one passes to the other side of the street. You cannot use 'one' to refer to it or any other pronoun, and visa versa.