Final answer:
Marxist theory posits that race is used by the capitalist class to divide the working class and maintain power, thereby hindering the workers' ability to unite and challenge the capitalist system.
Step-by-step explanation:
According to the theories of Karl Marx, as referenced in Greenstein's discussion, race plays a significant role in class relations under capitalism. Marxists argue that one of the tools used by the capitalist class to maintain power and suppress the working class is the promotion of racism and ethnic bias. The primary impact of race on class relations, as described in Marx's perspective, is to divide the working class. This division distracts workers from different racial and ethnic backgrounds from uniting against their common oppressor: the capitalist class. Hence, the correct answer is B. Divide the working class. This tactic prevents the formation of a united front that could challenge the status quo and potentially lead to a revolution, thereby perpetuating the capitalist system and the bourgeoisie's control.