Final answer:
Greta Thunberg was prompted to speak out because of her concern about climate change, influenced by Sweden's environmentalism and other activists, which led her to become a significant figure in environmental activism.
Therefore, option C is correct.
Step-by-step explanation:
The exigence that prompted Greta Thunberg to speak is c) Concern about climate change. Thunberg's initial actions, such as persuading her parents to reduce their carbon footprint and her solitary protests at the Swedish parliament, were driven by her deep worry for the environment and desire to effect change in global climate policies. Her passion for the environment was influenced by Sweden's strong ethic of environmentalism and the activism of American students on other social issues. Over time, her efforts have inspired a global movement, earning her recognition as Time's "Person of the Year" in 2019.