If I were you, I “would go” to a doctor. This is best represented by option A.
In this sentence, one person is advising or providing a suggestion to someone else. Inserting “will go” renders the sentence to be a command. Inserting the term “go” to the sentence would make it grammatically incorrect in that nothing is between the subject and verb. Lastly, if “went” were put in the blank, it would not make sense because it makes the entire sentence past tense.
It would make the most sense to use the sentence as follows: If I were you, I “would” go to a doctor. This provides a manner of concern and advice given to the initial person.