Final answer:
Approximately twenty percent of American adults reach the level-three stage of moral development according to Kohlberg's theory.
Step-by-step explanation:
The question is asking about the percentage of American adults that reach the level-three stage of moral development according to Lawrence Kohlberg's theory. Kohlberg identified three levels of moral reasoning: pre-conventional, conventional, and post-conventional. Subsequent research, including Kohlberg's, suggested that fewer than 25% of people ever progress to the post-conventional level, which is associated with higher stages of moral development, where morality is thought of in abstract terms and recognizes complexities such as the difference between legality and morality. Kohlberg's theory has also been the subject of criticism, particularly by Carol Gilligan, who noted potential gender biases in the theory and offered an alternative perspective on how men and women may approach moral problems differently.
Based on Kohlberg's findings, the correct answer to the given question is: twenty percent of American adults reach the level-three stage of moral development. So, the correct choice from the options provided is c. twenty.