This question is testing your knowledge about tenses. The word "if" requires a logical step that follows it immediately. In the case of choice B, the if is referring to an event in the past, so logiclly the follow-up step would have taken in the past as well. Let's examine carefully.

The first clause of B is "if the patient........(past perfect tense because of had known), second clause "he would have come in earlier (past progressive tense). The way to correct it is, "If the patient had known....then he would have gone to see the doctor". Otherwise, use just had known, which is just past progressive tense.

I think the key here is in case of subjunctive construction should be
Subjunctive + That + Clause (Clause should have the verb in base form)
Choice A exactly follows this pattern by using "known". Only confusion is presence of "had" after that, but here had is not used as a verb.

In choice B verb construction is "had known" which does not agree with the subjunctive construction.

what is the problem with (C) here kindly explain

B. if the patient had known the warning signs of canser, he would have come in earlier
C. if the patient knew the warning signs of canser, he would have come in earlier

The answer choice is 'B' because it is has a 'condition' and correctly uses the construction below:

(past perfect (1st) + 2nd (would +have past participle)

I'm still confused by the nuances between A and B.

Bad question
Both A and B are correct!!!