Science is a sort of news agency comparable ___________ to other news agencies.

The statement refers that Science and news agencies share similarities. Both of them share a common principle in terms of making the people more aware, so they are comparable in principle. Option (b) is correct. Option (a) is incorrect, because principally means mainly or chiefly. Option (c) is incorrect, because principal means the most important element or someone who is the head of a school. Option (d) is incorrect, because in spirit and form means a very close relationship which seems too farfetched.