Final answer:
Sentence 3 is the part of the evidence in the paragraph, providing an example of a successful zoo breeding program for Sumatran tigers.
Step-by-step explanation:
The sentence that is part of the evidence in the paragraph about the role of zoos in conservation is Sentence 3. This sentence specifically provides a concrete example of conservation efforts undertaken by a zoo, stating "The National Zoo in Washington D. C. has bred Sumatran tigers, boosting a population that was once only seven hundred in the world." It offers factual data to support the claim that zoos are crucial in saving endangered species by illustrating a successful breeding program for the Sumatran tigers.