Because the bamboo plant on which the panda feeds is now scarce naturalists are worried about their chances of survival

I'm not really sure what this question is about - is it a true or false question? If so, then the answer is true - given that there are fewer bamboo plants in the world (because they cannot grow as fast as pandas can eat them), there won't be enough food for pandas to thrive, and thus they won't survive for much longer.

If you just need to write the words necessary to complete the sentence, then 'on which,' 'is now scarce,' 'about,' and 'their' are the correct answers.