Final answer:
The Loisels lie to Madame Forestier to conceal the fact that they have lost her necklace, driven by fear of embarrassment and severe consequences, which results in them incurring significant debt. Therefore, the correct option is B.
Step-by-step explanation:
The Loisels lie to Madame Forestier because they want to hide the loss of the necklace. The story "The Necklace" by Guy de Maupassant highlights the pitfalls of vanity and pride, evident in the characters' decision to replace the lost necklace rather than confessing the truth to Madame Forestier.
Their motive is rooted in fear of embarrassment and the possible consequences they might face if they admitted to losing something that appeared valuable. This leads them to live a life of hardship and debt to pay for a replacement that matches the original's perceived worth. It's a tragic turn of events as the lie compounds their difficulties, stripping them of the modest comfort they once enjoyed.