Q. I was ill, so I went to see . . . . . . . . doctor who has a clinic beside my home.

Explanation by: Mr. Dubey

We use the definite article 'the' when the noun is followed by a clause or phrase which identifies it. In the given sentence, the 'doctor' is identified with the clause 'who has a clinic beside my home'. Option C is correct here since we are referring to a specific doctor.