[upon learning of lucy’s engagement, the narrator describes elinor: ] elinor’s security sunk; but her self command did not sink with it. what can you infer from the narrator’s description?

The correct answer is the second option. While Elinor does not feel secure, or safe, she maintains her self control. This means that she does not actually portray the emotions she's feeling, but instead appears fine. "Letting your guard down" signifies that you're showing emotion, which Elinor does not do. Instead, she remains composed, and keeps her guard up. 

By by showing how hard it was for the sisters to feign grief