The correct answer is: Their decision to lie leads to their great financial difficulties.
In this passage, Madame Loisel faces a moral dilemma after losing the borrowed necklace. Her decision to replace it with a similar one without telling Madame Forestier causes them to undergo significant financial struggles. This choice directly influences the plot by leading to years of hard work and poverty as they struggle to pay off the debt incurred from replacing the necklace, ultimately culminating in the irony of their situation and the story's twist ending.