Thanks for the question! If you understand why (B) is right, then you should know what the flaw in the argument is, and why (D) doesn’t get at it. But let’s take a look at what (D) says. It says that the flaw is that the argument presumes for no reason that educating workers always results in an increase in their productivity. Well, that can be diagrammed
Educating workers —> Increase in productivity
Does the argument assume this? No, it definitely doesn’t have to assume this at all. And if it doesn’t assume that in the first place, then that can’t be the flaw.
Hope this helps! Feel free to ask any other questions that you might have.