The correct word to use here is 'must' - making Option C) the correct answer. This is because it is the appropriate word to use when urging people towards a cause, which is what is being stated in the original sentence - saving tigers. Further, must in itself is the only appropriate modal verb to fit in the blank - have is incorrect; could expresses a mere possibility and shall is overly formal and mostly used with future conjugations. All other options are incorrect.