At the climax of the story, Mathilde learns:

Choose the best answer from the options below:
A her husband has been cheating on her all along
B her husband had borrowed the money to buy a necklace
C the necklace that she and her husband had worked so hard to pay for was just cheap paste jewelry
D the necklace her husband had bought for her was fake

C the necklace that she and her husband had worked so hard to pay for was just cheap paste jewelry

Based on the given options, the correct answer is:

C the necklace that she and her husband had worked so hard to pay for was just cheap paste jewelry.