I know (1)/a doctor (2)/you are referring to (3)./No error (4)

Answer

2

Explanation

the will replace a because -
here, we are talking about a particular/definite/specific doctor.
The sentence can also be written as -
I know the doctor `underset("Rel.Pro.")underset(darr)("who")` you are referring to.
Look at the examples given below :
The pepole I met there were friendly (Definit - those over these)
The people `underset("Rel.Pro.")underset(darr)("whom")` I met there were very friendly.