Thanks for the question! We’re looking to complete the argument here, so let’s take a look at that argument. We’re told that human beings can live happily only in a society where love and friendship are the primary motives for actions. We’re also told that economic needs can be satisfied without a society where love and friendship are the primary motives for actions. In essence:
Happiness requires X Economic needs does not require X
And what does this tell us? If we don’t have X, we could still have economic needs, but we couldn’t have happiness. In other words, we could satisfy our basic economic needs without obtaining happiness. This is what (D) tells us, and this is what logically completes the argument.
Hope this helps! Feel free to ask any other questions that you might have.