Correct Option: B

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.
The correct word is filled in the above given sentence :-
Science is a sort of news agency comparable in principle to other news agencies.