The correct option is A It is important to observe traffic rules.
The final sentence should be imperative in nature. Option B means that something is important so that traffic rules can be obeyed, which is not an order. Option C provides a part of the predicate before the subject, hence it is wrong. Option D implies a justification of some statement, hence it is not imperative either. Therefore, option A which mandates that traffic rules should be followed is correct.