Answer: B) The girl ran across the street, and she got the mail.
Step-by-step explanation:
A compound sentence is a sentence which consists of at least two independent clauses.
When we use a conjunction to join two independent clauses, we put a comma before it. In this case, the conjunction 'and' is used to join the independent clauses 1) "the girl ran across the street" and 2) "she got the mail." These two clauses form a compound sentence.