Answer:
Topic: a.
Main idea: c.
Explanation:
The fact that Martha died, and that pigeon´s particular story is presented as an example to present the topic, which is how passenger pigeons became extinct. Martha´s experience, as well as the details about large-scale pigeon hunting during the nineteenth century in America, are presented as evidence of the main idea, which is that by the beginning of the twentieth century became extinct by hunting and colonization of otherwise uninhabited areas.
