The answer is letter D: proletariat. Marx and Engels come from an historical analysis of oppression caused first by the monarchs, and then the bourgeoisie. The proletariat is said by Marx to be fighting the bourgeosie since the beggining of their existence, and by acknowledging that the workers should be united and fight all the oppression suffered.
The Communist Manifesto has harshly criticized the capitalism, and pointed the proletariat as the only capable social class to rise up and lead the government.