'one' is the problem in A. Why? 'it' has been consistently referring to the 'condition'. 'one' also refers to the 'condition' when a 'it' could be used - as in C - thus making 'one' intrusive in A.
I prefer C.
More about 'one' as a pronoun:
When used as a pronoun, one has some distinct usages. In American English, some of these usages are substituted for personal pronouns because the pronoun one reads a bit stuffy or pretentious. However, at times, using one is appropriate as an impersonal, reflexive or possessive pronoun. Using one is preferable to using the pronoun you. In formal writing, especially, one should not address the reader as you. As illustrated in the last example, one can be used in the plural when it represents a group of things or people.
EX: One of the operations was cancelled due to budgeting.
This usage can represent the writer, a collective group like the writer or a class of people.
EX: If one fails to assess the data correctly, the input is compromised.
3.Reflexive and Possessive Forms
In American usage, these forms of one, oneâ€™s and oneself, can often be replaced by other pronoun forms, such as his or her.
EX: One must learn from oneâ€™s (his) mistakes.
EX: If one misjudged the next cliff down, one could hurt oneself (herself).
Note: Both examples of personal pronoun usage are restricted to spoken English since gender bias occurs with himself or herself.
EX: The ones speaking out for anti-war policies represented a minority of those at the rally.
Note: Ones represents a common group of people while those refers to everyone attending the rally.