(a),(b),(c) are correct options.
From the beginning of 20th century onwards there has been Inequality in US. This is mainly because of the US tax and labour policies . In the beginning , top 15% of the country's wealth is owned by the top 1% of the rich class . Another study shows that top18% of wealth is owned by rich class of the soceity. Introduction of income tax which did not result in redistribution aggravated the situation. In the pretext of upbringing middle and lower class population in US , the policies only helped top class population in US at that time.
Significantly during great depression, the income inequality came down drastically since the share of income from top class has reduced .But from 1970's onward inequality againnrose. The Gini index a measure of income distribution among population rose above 40 during this time and the inequality has continued over time. Less progressive tax and transfer policies is the reason for inequality during this time
US , unlike other nation's consistently shows income inequality due to relatively less regulated economy, high immigration and low skilled workers