Answer:
The implicit detail is that Jilly got a bad grade.
Step-by-step explanation:
When something is implicit, it's suggested or implied but not clearly said. The opposite of this is something explicit - stated clearly and in detail.
Here, we have Jilly, who frowned after seeing her grade on the test. When someone frowns, they are experiencing negative emotions, such as sadness, anger, or disappointment. We can guess that Jilly is disappointed by her test results, that she got a bad grade. That is the implicit detail suggested by this sentence.