Hey there!
Your answer would be B, implied metaphor.
Here, we see four answer choices. An epic simile is a simile in the same form except many lines in length. An implied metaphor is not one that is explicitly stated, but is there. Imagery is using word choice to describe something which forms an image in the reader's head. Personification is using human actions to describe something which is not human.
We can tell this is an implied metaphor by this: "Whenever she is put in a situation that might reveal her true personality, she withdraws into a shell, where she feels safe and hidden." The most important part here is "she withdraws into a shell". There is no actual shell, of course, however, this is just making a comparison with her drawing into herself. The comparison is not explicitly stated, but it is there.
Hope this helps!
