According to British English, “-‘s” is added to a word and is used to refer to a person’s home or shop–http://oald8.oxfordlearnersdictionaries.com/dictionary/s_4. However, in this particular test, I am wondering why there is no “-’s” added to the “doctor”.
Nobody seems to be answering your question. As a native English speaker from the U.S. I can only say that doctor or doctor’s would be acceptable to me, although I usually say doctor. I looked up the google ngram, which can be a valuable tool since “what is correct” is usually defined by “what most people write”. But unfortunately, it was inconclusive.