The correct option is B Colleague
'An associate in an office or institution' is called Colleague, Option C. Option A, 'Companion' is a person with whom we spend a lot of time. Option B, 'Ally' is a country formally cooperating with another for military or other purposes. Option D, 'Accomplice' is a person who helps the other commit a crime. Thus, options A, B and D are incorrect and the correct for the given phrase is Option C, Colleague.