The correct answer is ‘doctor’ The only time you would capitalize the word is when it begins the sentence. Like a fill in the blank, Ex. ______ help their patients out of office. In this case it would be Doctor. Otherwise, only names are capitalized. ***Do not capitalize all the letters DOCTOR. This is not good and will most likely be marked wrong. The IELTS does follow the same capitalization rules as the general English language. No surprise there : ) Keep up the good studies.