If the pain in your knee continues, l advise ... a doctor. a. you see b. to see c. you to see

The correct option is b. to see. So the complete sentence would be: "If the pain in your knee continues, I advise to see a doctor." However, for a more natural phrasing, it could also be structured as: "If the pain in your knee continues, I advise that you see a doctor." But from the given options, "to see" is the correct choice.