B: "I can't believe you sewed so perfectly on your first try," Mom said, patting Juan on the back.
Explanation:
The sentence that confirms that Juan doesn't have a lot of experience sewing is sentence B. Here, it's explicitly stated that this is Juan's first attempt sewing (... <em>on your first try</em>). However, despite his lack of experience, Juan sewed perfectly. The rest of the options do not contain this type of information, which is why they are incorrect.