The correct answer is option 2, Karl Marx. Karl Marx is the author of the famous book "Das Kapital". "Das Kapital" is a seminal work in the field of political economy and is considered one of the most influential books in Marxist literature. It was first published in 1867 and analyzes the capitalist mode of production and the exploitation of labor within the capitalist system.

Now, let`s dissect the other options:

- Option 1: Adam Smith is not the author of "Das Kapital", but he is a renowned economist and philosopher known for his book "The Wealth of Nations", which is considered one of the foundational texts of modern economics.

- Option 3: Rousseau is not the author of "Das Kapital". Jean-Jacques Rousseau was a philosopher and writer whose works focused on political theory and the social contract.

- Option 4: Voltaire is also not the author of "Das Kapital". Voltaire was a philosopher and intellectual of the Enlightenment era, known for his advocacy of freedom of speech and separation of church and state.

In summary, the correct answer to the question is option 2, Karl Marx, as he is the author of the famous book "Das Kapital".