The most common way to start a sentence is with a subject, making the correct answer B.
An example would be a simple sentence "He broke the window" where "he" is the subject.
Your answer is A.
This is why:
At the end of this short novel, the audience learns that the Necklace was fake! If Mathilde would have come clean with the fact that she lost it, she wouldn't have had to do all that work and make herself old.
