We need to parallel the conditional reasoning in this short passage. The conditional diagrams in this passage look like this:
Premise 1: People who are indifferent to their environment - > pollution is a problem (A - > B)
Premise 2: Pollution - > Nature's balance is harmed (B - > C)
Conclusion: People who are indifferent to their environment - > Nature's balance is harmed (A - > C)
Now let's find the answer choice which parallels this reasoning.
Answer choice A does the exact same:
A - > B and B - > C, therefore, A - > C
Answer choice D on the other hand does the following:
A - > B and B - > C, but then it incorrectly (and most importantly, differently) concludes the following, therefore C - > A. It reverses the conditional diagram.
I hope this helps. Please let me know if you have any other questions.