Answer:
The sentence that contains an idiom is "Margo was over the moon when she hit her first homerun in softball."
Step-by-step explanation:
The first and last sentences are facts, and the second sentence contains a metaphor, but not an idiom. The third sentence is the sentence that contains an idiom because an idiom is a phrase that is well known but does not have a literal meaning, and "over the moon" is a well-known phrase with no literal meaning.
Hope this helps!