Malthus and Marx . a. disagreed about the severity of overpopulation, poverty, and social upheaval. b. disagreed about the root causes of overpopulation, poverty, and social upheaval. c. held the same beliefs but worked on different issues. d. worked together to form their theories.

Option(b) is correct

Explanation:

According to the concept of Malthus and Marx , they had different opinions and ideas for source of poverty , misery, overpopulation and other social aspect in society. Malthus believed that poverty and low food supply, hunger are caused by overpopulation and Marx stated that these situations are result of capitalist economy.

Other options are incorrect because theories generation by combining, didn't have similar beliefs and did not concern severeness in issue of overpopulation, social poverty etc.Thus, the correct option is option(b).