Answer:
The best option is indeed letter D. Controlled burns duplicate natural fires that have always helped prairies prosper, so they are valuable to our natural landscape.
Explanation:
Even though all the options provided are closely related to the topic sentence at the beginning, we were asked to choose the one that would be most appropriate to start the conclusion. A conclusion has the purpose or restating what was previously claimed in a shorter way. Basically, it should claim the author's opinion again, but in fewer words. The conclusion should never present new information, otherwise it will be just and extent of the text's body.
The claim made in the topic sentence is basically that controlled fires are a worry to some people, but that they help the environment. We assume then that, throughout the text, the author has presented arguments that prove people they shouldn't worry about controlled fires for several reasons and that prairies actually benefit from them. Letter D presents a brief restatement of all that: if controlled fires duplicate natural fires, they are not really polluting and, therefore, there is nothing to worry about. If they help prairies prosper, they are helping the environment. If we were to choose any of the other options, we would be choosing the explanation and arguments that would be better presented in the text's body. For the conclusion, letter D is the best choice.
