Answer:
b. Sentence 6
Step-by-step explanation:
I'm assuming that your question is asking about which sentence is proof that zoos are needed to protect animals, because that seems to be the main idea of this paragraph.
If that's the case, I think that the answer would be b. (Sentence 6) Many zoos have breeding programs that allow species to multiply and thrive. Those breeding programs would help protect animals (such as endangered species) because those programs would give those animals a safe habitat to live and expand their population once again so they don't die off.
hope this helps :)