Malthus and marx:
a. worked together to form their theories.
b. agreed about the root causes of overpopulation, poverty, and social upheaval.
c. disagreed about the root causes of overpopulation, poverty, and social upheaval.
d. held the same beliefs but worked on different issues.
e. disagreed about carbon footprints versus socialism.

c) disagreed about the root causes of overpopulation, poverty, and social upheaval.

Explanation:

First we will explain the vision of Malthus and Marx regarding overpopulation:

The Malthusian theory explains the relationship between the growth in food supply and in population. Malthus believed that population increases faster than food supply and therefore, if there is no control of population, this situation (overpopulation) will lead to famine and misery. Malthus believed that people had as many children as they could and this would only make overpopulation worse.

On the other hand, Marx believed that people understand the consequences of having many children, and, since parents want what's best for their children, as their income grows bigger, they will actually have less children because this would mean their children will actually get better opportunities for development.

Now, on poverty:

From Malthus beliefs on overpopulation we can see that he thought poverty was an outcome of having too many people populating the world. However, Malthus actually believed that these outcomes were actually God's desire.

On the other hand, Marx develop an entire theory on poverty. In very general terms, he stated that poverty was the consequence of capitalism, therefore it was consequence of what human beings are doing rather than God's desires.

And on social upheaval

Considering both previous topics, Malthus considered that the overpopulation would lead to misery and famine and this would be the cause of massive social upheaval. While Marx would state that the social upheavals would arise as a fight of classes where the people on the lower classes of society would start a revolution against the people with more power and this would be the cause of social upheavals.

After this analysis we can see that Malthus and Marx disagreed about the root causes of overpopulation, poverty and social upheaval

*They did not work together to form their theories, and their theories actually opposed.

They didn't agree on the main causes of the topics explained below and they neither held the same beliefs (as we already wrote).*