Thanks for the question! This is a one that you could diagram.
Premise: Motive influences all human actions —> Arise from self-interest
Conclusion: Self-interest is the chief influence on human action
However, we’re looking for a flaw here, and clearly, the flaw is that there might be a chief influence on human action that doesn’t influence all human actions, just most of them. But that one is the chief one. That’s basically what (B) says, which makes it the correct answer.
Hope this helps! Feel free to ask any other questions that you might have.