Option B: 'Colleague' is 'a person with whom one works in a profession or business'.
It means the same as the given phrase and hence can substitute for it. So, option B is correct.
Option A: 'Comrade' is 'a colleague or a fellow soldier or member of the armed forces'. It almost means the same as 'colleague' as a 'fellow member' but it does not refer to a workplace as mentioned in the phrase. Hence it cannot substitute for it and option A is wrong.
Option C: 'Assistant' is 'a person who helps in particular work'.
Option D: 'Contemporary' is 'a person who belongs to the same time and age as another', 'belonging to the present or current times'.
These words do not mean the same as the given phrase and hence cannot substitute for it. Hence options C and D are wrong.